When I was a boy, we seldom had the right tools or materials at hand for any project, including adhesives. If a dried-up bottle of mucilage or white glue wouldn’t work, we were out of luck. Once, my father had the ingenious idea of pasting up a homework assignment with a goo of flour and water—”After all,” he reasoned, “isn’t that the same as wallpaper paste?” You can guess the outcome.

Since that time, dozens of adhesive types have come on the market for the perfect stick, along with new formulations that are easier to use and, when selected appropriately, last a long time. Like most things in building construction, there is no one-size-fits-all glue. The best approach for restoration work is to have a well-chosen selection to meet most needs. To help sort out this broad industry, here’s a primer on the basic adhesives types you’ll find at good hardware stores, and what you’ll need to know about them for bonding around your old house.

Glue Basics

Generally speaking, adhesives—glues (traditionally made from natural sources) or cements (frequently rubber-based)—are all liquids that solidify to bind together similar and dissimilar materials. While some adhesives can be used for a multitude of applications and conditions, you will get the best results if you carefully match the characteristics of the adhesive to the requirements of the project. For example, to repair the handle on your favorite coffee mug, you’ll need a glue that 1) sets up quickly while you hold the handle tightly in place; 2) is strong enough to hold the weight of 8 ounces of scalding hot beverage; 3) is relatively invisible; and 4) is waterproof to resist dissolving in dish water. Woodworkers look for other qualities, such as quick or slow set (for careful assembly) or perhaps weather resistance (for exterior use). Some of the less obvious but equally important characteristics are often defined in this way.

Adhesion: The bond between a material’s surface and the adhesive. Adhesion is often mechanical (where the adhesive interlocks with tiny pores and crevices in the material), but it can also be molecular.

Creep: The tendency of an adhesive to slowly stretch, especially under stress.Cure The length of time an adhesive takes to reach (or almost reach) its ultimate strength, the point at which the repaired object can be used safely. Flashing off The process of encouraging the escape of solvents in solvent-based adhesives so as to speed setting—for example, pulling apart two pieces just after applying adhesive, then reassembling them minutes later.

Open time: Also called working time and assembly time, this term refers to the period you have to assemble and clamp parts before the adhesive sets up or loses its ability to work properly.

Reversibility: The ability of the adhesive to be softened, and the joints disassembled, typically by applying steam, water, heat, or solvents. Reversibility is important when repairing expensive furniture.Tack The initial stickiness or bonding of an adhesive; good initial tack helps with assembling parts.

Waterproofing/water resistance: A waterproof adhesive can be immersed in water and still work; adhesives labeled water resistant are made for exposure to water and humidity, but they may fail if immersed in water.

Adhesives often create a bond that is tougher than the materials being joined, but not without following a few basic rules for good results. Parts and joints should be tightly fitted, then clamped or weighted for successful adhesion. Simply combining components in haphazard fashion is a formula for failure. Never depend on the adhesive to fill voids left by loose joints or missing pieces. When regluing joints, remove as much of the old glue as possible to create a tight joint; new glue does not adhere well to old glue.

White Glue

All-purpose white glue is a must-have for interior craft projects. Made with polyvinyl acetate (PVA), the first and most widely used synthetic resin for wood adhesives, it hardens through the evaporation of water. Therefore, white glue can be thinned and cleaned up with water, is safe to use around children, dries clear, sets fast and, after presoaking, washes out of clothing. White glue is the all-round best adhesive for paper, cardboard, wood, fabric, and a multitude of craft materials not subject to damp conditions. Depending on the temperature, white glue has a working time of five to 20 minutes and cures in an hour or two. I once watched my wife successfully reconstruct a centuries-old, Native American pot broken into at least 60 pieces with all-purpose, white glue. It is reversible for about one month and can be softened with steam or warm water.

Yellow Glue

Like white glue, yellow glue is based on polyvinyl acetate but formulated to be faster setting (five to 15 minutes), more viscous (to reduce ooze under clamping), and easier to sand, making it the standard wood adhesive for most carpenters and home hobbyists. Yellow glue cures overnight, cleans up with water (while wet), dries to a yellow, and is nontoxic. After hardening, excess glue can be chiseled, scraped, or sanded off surfaces. Unless otherwise specified, most products are not water resistant and should not be used for exterior projects subject to moisture or high humidity. Compared to white glue, yellow glue is generally less prone to creep but still enough so that it is generally not recommended for structural applications for this reason. Since yellow glue does not absorb stain, fastidious application and cleanup are essential to prevent the glue from showing through stains or sealing the wood surface prior to staining. Yellow glue is also sold in dark browns designed for use on dark woods like walnut or mahogany where the yellow version might create a thin yellow line.

If you need glue for exterior woodworking projects subject to moisture, humidity, and temperature changes, look into ASTI Type I water resistant and ANSI Type II waterproof wood glues. In general, these glues handle just like ordinary yellow glue, but they are rated either moisture resistant or waterproof according to stringent tests conducted by the American Society of Testing Engineers. When there’s question about use, always read the manufacturer’s recommendations. Moulding and trim glue is a polyvinyl acetate formula specifically sold for carpenters and woodworkers who need a fast-setting glue that will not run or sag. This glue needs very little clamping time (five to 10 minutes), and resists running onto other surfaces—for example, down from a crown molding onto gypsum wallboard. Moulding and trim glue is for interior use only. It cleans up with water while wet.

Polyurethane Glue

Polyurethane glue (sold under a variety of trade names like Gorilla Glue or PL glue) is a relatively new face on the “adhesive block,” appearing on the consumer market only in the last 10 years. Unlike the evaporative action of white and yellow glues, polyurethane glue is chemically reactive, meaning that it cures by reacting with another liquid—specifically, the moisture present in the air or in substrates like wood. This curing process makes polyurethane glue much less sensitive to environmental conditions and well-suited for the temperature swings and high humidity of outdoor use. Polyurethane glue has a working time of about 15 minutes and, though relatively expensive, is efficient to use because it foams to three or four times its original volume. Polyurethane glue is also extremely strong and highly polar in nature, giving it great ability to adhere to dissimilar surfaces. For example, I recently joined bare wood to painted wood with success. A light spray of water just prior to applying the glue improves the bond. Polyurethane glue dries to a light amber or tan, and can be painted, stained, and sanded. It cleans up with mineral spirits while wet, but must be scraped or sanded off the surface. Always wear gloves when working with polyurethane glue; once dry, it is difficult to remove.

Epoxy

Though many OHJ readers are very familiar with the working characteristics of epoxy products like wood consolidants and fillers, epoxy adhesives are worth mentioning here because they fill many niches not covered by other adhesives. Epoxy is a two-part, thermosetting (heat-reaction curing) compound that, when used correctly, creates a strong, waterproof bond—that’s why it’s so popular in the marine industry. Working time can vary from an hour or more to just a few minutes depending upon how the manufacturer has formulated the product, what the environmental conditions are, and how the epoxy is employed. Epoxy adhesive products are remarkably diverse, ranging from paste fillers that are formulated to look and tool like wood, to water-white adhesives for mending glass, to structural adhesives that can be used for load-bearing conditions. Epoxy is relatively expensive and the unmixed components (resin and hardener) must be handled with care. Workers using epoxy should wear gloves and provide adequate ventilation or wear an appropriate respirator. Epoxy can be painted after sanding to create a mechanical bond.

Animal Glue

Historically, hot animal-hide glue was the only game in town. It was used to assemble furniture and veneers and is still the choice of fine furniture restorers because it is strong, needs little clamping, and is reversible—joints held fast with hide glue can be steamed apart without damage to the adjacent wood parts. The original hide glue is a specialty product and worthy of an article unto itself. There is now a ready-to-use hide glue available on the hardware shelf that requires no heating, or mixing, does not have a rancid odor and is reversible, and creep resistant. This new product is still based on an animal protein formula but is improved by chemistry to conform to modern demands. This is a great glue for furniture repairs. If, however, you are faced with repairing a valuable heirloom piece of furniture, say, an 18th-century Windsor chair, we recommend you consult a furniture conservator first.

Cyanoacrylate

Cyanoacrylate glue (sold as Crazy Glue or Superglue) is an acrylic resin that polymerizes in the presence of water. It was formulated by Eastman Kodak in 1958, but wasn’t available on the retail market until the 1970s. Most of us have had mixed results with cyanoacrylate because we failed to understand its attributes and how it works. Cyanoacrylate sets up very quickly—in fact, almost immediately—reaching full strength in about two hours and full cure in about 24 hours. It is a tenacious adhesive that works well on nonporous surfaces and surfaces that contain a trace of moisture. It’s great for attaching small plastic, metal, or glass pieces and less useful for porous materials. (Interestingly, cyanoacrylate is now commonly used on the human body for suture-free surgery.) For the best results on nonporous surfaces, apply the least amount in the thinnest possible layer and follow with pressure until the initial set takes place, usually in about one minute. Given these quick-setting properties, always have all pieces and parts at hand before applying the glue. When setting, cyanoacrylate exudes acetic acid that smells strongly of vinegar. Since it sticks tenaciously to skin and can only be removed with acetone (a strong solvent) it is not appropriate for use by children.

Contact Cement

Contact cements are a group of adhesives that, once applied to a substrate, continue to remain sticky or tacky, allowing them to adhere to themselves when assembled to another cement-coated substrate. Contact cements are used to bind materials that need an instant set, like laminate counter tops and rigid sheet goods, but are not appropriate for most woodworking projects. Formulations vary, but many products are based on neoprene rubber. Unlike glues, where the parts are pressed together while the glue is wet, contact cement is applied to both sides of the objects to be glued and then allowed to dry before assembly. This means that the parts must be carefully positioned because, once set, they cannot be pulled apart. Evenly applied pressure creates a stronger bond. Formerly only available in smelly, solvent-based formulas, contact cements are now made in more environmentally friendly, water-based versions.

Tile Adhesives

Flooring adhesives, as well as non-cement-based products for ceramic tile installation, usually come premixed in a bucket and are troweled into place. There are many types of flooring, carpet, and tile adhesives; before choosing one always refer to the manufacturer’s recommendations to avoid compromising the installation of expensive materials by using the wrong adhesive. Today’s general-purpose ceramic tile adhesives are often water-borne and based on rubbers like latex so that they achieve a fast initial tack (for holding tiles in vertical positions) and remain flexible and water resistant once cured. Though high-moisture areas or specialized substrates may require specialized products, latex thinset adhesive is often recommended for common tile installations.

Specialty flooring materials may have their own adhesive requirements. When my wife and I installed our new linoleum floor, the instructions were clear: Use the manufacturer’s proprietary adhesive or proceed at your own risk (all warranties were void if a substitute was used). Multi-purpose adhesives are usually for the installation of sheet vinyl and carpets. You’ll also find solvent-free adhesives, interior and exterior quality adhesives, stain-free adhesives, epoxy and polyurethane adhesives, latex adhesives, and asphalt-based adhesives. The good news is the solvent-laden, toxic flooring adhesives of the past have been replaced by more environmentally friendly versions, making the work site more tolerable, installation easier, and with water clean-up possible.

Stained cracks and suspicious shadows—the telltale signs of a delaminating plaster ceiling.

What old-house owner doesn’t have a cracking or sagging plaster ceiling in need of repair somewhere? Besides the normal wear and tear of living, plaster-and-lath ceilings are at the mercy of gravity, and they can take only so many water leaks and structural movements before they pull away from the framing. The good news is, it’s possible to repair and rescue them from further damage. We have successfully reattached many old ceilings by injecting adhesive between the plaster and lath where the keys (anchors) have broken away over time. Though variations of this technique are not new texts from the 1920s recommend liquid sulfur as an adhesive-we use modern materials that are easy to handle and inflict minimal damage to sound plaster. With these methods and good tool skills it’s possible to restore the integrity of plaster ceilings for many more years of service.

Sizing up the Ceiling

After correcting whatever problem made the ceiling loose in the first place (leaky roof, structural alterations), the first step is to assess how far the ceiling has pulled away. Gently push on the surface and judge the amount of play between plaster and lath. Hopefully, you will feel it move back into place like a jigsaw puzzle piece sliding into its own unique spot. If there are broken keys or debris in the way (see sidebar p. 54), the plaster will resist seating, and it will feel and sound “crunchy” akin to breaking eggshells or crushing popcorn. Do not force it back into place or more plaster may break. Instead, just gently encourage the plaster with the flat of your hand. If the separation is slight—say, between 1/4″ and 1/2″—good reattachment is likely because the plaster will usually push back into place solid and flat against the lath. If the plaster sags 1/2″ to 1″ from the lath, there is often too much debris (broken keys, years of silt) between the plaster and lath for success.

Holding a vacuum nozzle on the drill bit helps suck the 1/4" injection holes clear of dust.

Unless you can vacuum out all this debris from above, reattachment is probably not an option. Often we find that the worst part of the ceiling has too much debris, and we have to remove this section, reattach the edges, then infill the lost parts with new plaster. Plaster that is soft and crumbly will not hold up during the pushing and drilling and has to be removed as well. Removing areas of damaged plaster—particularly in a ceiling—will encourage more plaster to come down, especially if you use a chisel-edged tool. (We call this the domino-delamination effect.) To control this tendency we recommend carefully marking out your repair plan, then removing any areas with a sharp utility knife. Mark with a lumber crayon or pencil; a pen will bleed through your paint later on.

The holes you bore to inject the adhesive must be directly beneath the wood lath—not the spaces between the lath—so that the adhesive can bond to something solid. If you have removed any plaster, or dug out a crack for repair, then you can see the positions of the lath. In a reattachment-only repair, however, finding lath is basically hunt-and-peck. Sometimes, if the plaster is not too thick, you can stand back and look for ghosts or shadows of the lath showing through the finish coat.

Once you have a solid fix on one lath, assume that the rest of the lathing is 1 1/2″ to 2″ wide and spaced approximately 1/4″ to 1/2″ apart. If you have an infill area that needs reattachment at its edges, mark for injection sites 1 1/2″ to 2″ back from the edges. If you are only reattaching plaster, plan for injection holes every 3″ to 4″, no more than 6″ apart.

The 1/4" injection hole is an effective size for a good seal with the adhesive tube.

We bore injection holes with a 1/4″ carbide drill bit. Making these holes presents a second challenge because it is very important to bore only through the injection surface. For example, if you are reattaching from the plaster side, you must bore completely through the plaster, yet stop before going into the wood lath. Conversely, if you are reattaching from above, you need to bore through the lath, but stop before the plaster. By paying attention to the changing resistance of the materials on the drill bit, it’s possible to develop a feel for these different layers. After you have bored your injection holes, you need to vacuum out the debris and drilling dust. Older plaster is soft, so be careful not to suck it off the ceiling with the vacuum! Place your hand near the hole to gently support the plaster while you vacuum using the other hand. Do not push the plaster back up into place, however; the void will allow the vacuum to pull some of the debris out through the hole.

If you have access from above, you can vacuum more of the silt and debris. Remove keys that are visibly loose or broken as well. Use a wet/dry shop vacuum designed to handle the fine dust. Plaster will kill a household vacuum in short order. Next, wet the injection holes by either spraying down the lath with a squirt bottle or squirting up into the drilled holes from the plaster surface. Wetting encourages the adhesive to travel farther when it is compressed and will also slow the drying time slightly for a stronger bond.

Bonding and Shoring

Finally it’s time to inject adhesive. We use a water-based, latex product that is actually a vinyl floor adhesive. However, any good-quality latex or acrylic adhesive can work (for example, Liquid Nails or floor adhesive). You can purchase these products at construction supply houses and hardware stores in caulking tubes for small projects or five-gallon pails for large jobs. We use a caulking gun with the tip cut to fit snugly in our 1/4″ holes, and inject the adhesive until the plaster moves ever so slightly (one squeeze of the average caulking gun is usually enough). If you inject too much adhesive you will actually push the plaster off the ceiling. As you inject, follow along with a damp sponge to wipe away the excess adhesive that leaves the holes when you remove the nozzle.

Adding wire lath over wood lath improves an infill repair.

Complete your entire series of holes, then go back over the plaster with a clean, damp sponge to remove further glue residue. Follow the same process if you’re working on the lath side, leaving adhesive wipe-up as an optional step. Once we have injected adhesive into the repair area, we push the plaster back into place against the lath and secure it with forms. This step spreads the adhesive so that it bonds to a greater surface area. We use flexible plywood squares (1/2″ to 3/8″ thick) covered with a layer of sheet polyethylene. Do not underestimate the importance of this layer of poly; if you forget it you will glue the plywood to the ceiling and take all the plaster with it if you attempt to remove the form. (Sound like first-hand experience?) Last, we secure these in place with screws or wood shores running to the floor, then allow the adhesive to set up for 24 hours.

Screws save the time of fitting wooden shores and keep the work area clear, but they can damage the plaster and leave more holes to fill later. Shores are more appropriate for fragile, decorative, or museum-quality plaster, and we use them for both plaster-side and lath-side reattachment. The next day you can remove the forms. Where some of the plastic sticks to adhesive injected from the plaster side, simply scrape it off with a putty knife. The adhesive is still soft around the holes at this point, but has set up enough to remove the forms. On the plaster side, scrape off the dried adhesive residue with a drywall or putty knife, then use the corner of a putty knife to gently scallop out excess adhesive showing from the hole.

As with all good restorations, a successful repair is hard to see.

As it dries fully over another day or so (depending on heat and humidity) the adhesive will further retreat into the hole, leaving a clear space to fill. We usually fill the injection and screw holes with Durabond 45 (sandable), mixed to the consistency of peanut butter, because this product has a quick set-up time and dries very hard with minimal shrinkage. However, any vinyl paste filler or plaster/joint compound can be used for this step. Usually, it takes two to three applications to fill the holes flush with the ceiling, depending upon the product. Other than this, the ceiling is now reattached and ready for a skim-coat of plaster or a fresh coat of paint.

Peter and Noelle Lord, operators of Peter Lord Plaster & Paint, Inc., specialize in the preservation and restoration of historic surfaces and all plaster systems.

The strength, adhesion, and moisture resistance of epoxies makes them excellent wood adhesives for new exterior woodwork, such as these post bases.

Who hasn’t heard of epoxy? Epoxies have been used since the 1940s to manufacture a wide variety of products and, in the last three decades, have become a primary tool for repairing building materials, especially wood and concrete. While some companies tout their epoxy products as close to miracle cures that can mend broken metal or revive deteriorated wood, there are also architectural conservators who believe that epoxies do more harm than good. To make things worse, there are many products on the market that are used similarly to—and often confused with—epoxies. All this can leave old-house owners scratching their heads. Is there a practical, middle ground where epoxies can be used to save exterior woodwork successfully? And just what is this epoxy stuff? A look at the nature and growing applications of this remarkable chemical repair technology can help answer these questions.

Epoxy Basics

Epoxy belongs to a broad family of thermosetting compounds—in simple terms, plastics that cure by heat reaction. Widely used epoxies like adhesives and casting materials are commonly made up of two liquids: an epoxy resin and a curing agent called a hardener. When the resin and hardener are mixed together, a chemical reaction takes place and the resin transforms into a solid mass. During the reaction, single molecules (monomers) of the epoxy resin and the curing agent combine to form chains of molecules (polymers). As the mixture cures, the chains grow longer, the solution gels, and then it hardens.

Epoxy materials are very adaptable. The chemists who design and formulate epoxies can give them a wide range of pre-cure and post-cure characteristics. It all depends on which epoxy monomers, curing agents, solvents, and fillers they add. The pre-cure mix can be thin as water to penetrate porous materials, or thick and viscous to stay put on vertical surfaces. The cured epoxy can be nearly as hard and brittle as glass, or almost as soft and flexible as rubber bands. Even the rate of cure can be adjusted to meet very specific needs. An epoxy that takes hours to gel can penetrate deeply into porous wood, while products that set up in minutes can be good for adhesives and floor coatings where the area must be put back into service as soon as possible. Many epoxy systems also contain additives, such as plasticizers to make them more flexible, organic solvents to make them more spreadable, and fillers, such as sand, to add bulk and reduce costs. Mixing in pigments adds color.

Since epoxies bond exceptionally well to a wide range of materials, they make excellent adhesives. Good bonding also means that epoxies are useful for making composite materials. For example, we can reinforce a repair by layering epoxy and fabric so that the resulting composite bonds to its surroundings, or by mixing the resin with powdery fillers or chopped fibers to make a paste that fills voids.

Epoxy fillers are two-part products composed of resin and hardener that must be thoroughly mixed before use.

What’s the point of all this adaptability? It makes epoxies compatible with the characteristics of traditional building materials—from stone, glass, and concrete to terra cotta and wood. For example, epoxies designed for wood repairs are specifically formulated to match the strength of wood and be flexible enough to move with its cycles of contraction and expansion. In fact, several manufacturers offer kits of epoxies specially formulated for such repairs that include one or more of the following specialized product types:

Epoxy consolidant: Liquid resin formulated to soak into fibrous materials, such as wood. The amount of penetration depends mainly on the dryness of the wood. Other technical considerations, such as capillarity, interfacial tension, molecular size, temperature, consistency, and gel time also play important roles.

Epoxy primer: Liquid formulated to prepare a surface for good adhesion of another material such as an epoxy paste filler.

Epoxy paste filler: Adhesive paste composed of a two-part liquid epoxy that is similar to consolidant. Manufacturers blend in powdery thickeners to give each paste a consistency that ranges from mayonnaise to stiff mashed potatoes. Other fillers give the cured paste the strength and flexibility characteristics of wood. You can even formulate your own paste by starting with epoxy resin, then adding familiar materials like sawdust and a little corn starch. Commercially prepared materials, however, are much more consistent and reliable.

Don’t confuse these products with epoxies formulated for other uses, such as five-minute adhesives, bar top coatings, or paints. Moreover, don’t assume that a product with two parts or a resin is epoxy. True epoxy products have become so common and popular that, in some conversations, the word epoxy is used generically to mean any resin applied to repair wood—even when the resin is not epoxy. Repair products like the following are sometimes caught up in this confusion.

Common auto body filler: These products, typically based on polyester resin, are used to fill dents in sheet metal, so they are usually designed to be fairly hard and inflexible when cured—not a great match with the inevitable movement of wood caused by changes in moisture content. Like polyester resins in general, their chemical “curing” reaction is set in motion by mixing in small quantities of catalyst, rather than the hardener used with epoxies.

Polyester wood filler: These products are designed for wood, but based on polyester resin. For example, one company offers a consolidant that is poly-ketone resin in an acetone-methane solvent. The solvent promotes penetration, but only the resin remains in the wood; the rest evaporates. The companion product is a two-part paste made of polyester resin and fillers.

Cementitious wood filler: This wood filler and repair system is based on a special cement, acrylic latex polymer, fillers, and fiberglass cloth. One manufacturer says its product’s high adhesion and flexibility prevent loosening due to wood movement. Unlike epoxies, it can be applied to damp wood and, when cured, allows water vapor to pass through thin sections.

The flexibility and tenacious adhesion of wood-epoxy products makes them ideal for outdoor repairs.

Wood Repair Methods

There are two common methods for repairing wood with epoxy: decay consolidation and decay removal. When wood decays, it becomes softer, weaker, and more porous than the surrounding sound wood. In the decay consolidation method you saturate the porous, decayed wood with liquid consolidant, which later hardens within the wood. It is important to make sure that absolutely all of the decayed wood is saturated, right down to and into the surrounding sound wood. Unfortunately, this is difficult to do since you cannot see where decayed and sound wood meet.

The alternate method is to remove all of the decayed wood. Then you prime the exposed surface of the sound wood with an epoxy primer, and fill the void with epoxy paste filler. It is sometimes easier to get more effective results with this method because the newly exposed surface of sound wood is much more consistent than the interface of the decayed and sound wood. Remember that you must also determine and control the source of the moisture. If a window sill is decayed due to an overhead gutter leak, you must fix the gutter along with the sill, or the sill is likely to decay again. Another point to bear in mind is that epoxies are, for the most part, not reversible. What makes wood-epoxy materials so effective from a practical point of view is their ability to penetrate deeply and adhere tenaciously, but this ability is also what makes the repair difficult to reverse. Since treatment reversibility is a key tenet of building conservation philosophy, this characteristic is sometimes required for work on historic landmarks or important buildings and artifacts—artisan-carved paneling or furniture, for example.

Most old-house owners, however, do not live in museums, and very often they can strike a balance between the shorter maintenance cycles of totally reversible treatments—the philosophical ideal—and the longer maintenance cycles of less reversible treatments. Longer maintenance cycles are particularly important for woodwork in remote locations, such as roofs, steeples, and high towers, due to the high cost of access. This not only reduces the cost of maintenance, but it has also been demonstrated to preserve more historic fabric on at least one national landmark building over the years. Balancing reversibility and sustainability should always be a concern, leaning toward reversibility with important historic buildings.

Keep in mind that there is nothing magical about epoxy that automatically makes it better. Traditional repair methods (such as wood dutchmen or splices) and modern methods (such as part replacement) should always be considered along with wood-epoxy repairs. Select the method and materials that meet the needs of your repair and the goals of your situation.

John Leeke, author and longtime OHJ contributor, is a preservation consultant based in New England. Contact John on MyOldHouseOnline.com.

The materials for average epoxy repairs are simple—common hand tools and containers of A and B fillers—but they can solve problems throughout the exterior and interior surfaces of an old house.

Many of the techniques that form the core of historic preservation are actually traditional tools, methods, and materials developed over centuries for the maintenance and repair of existing buildings, but not all of them. The modern restoration movement also makes active use of the full arsenal of modern building technology, such as power tools for cutting stone, or lasers and computer software for analyzing problems. This includes the use of man-made materials and compounds, and one of the most versatile and effective materials in this arsenal is epoxy technology.

Epoxies have continued to grow in their popularity and versatility over the last three decades, but many old-house restorers are still not clear about all these materials can do—and perhaps just as important, what they cannot do. It doesn’t help that there are many epoxy products on the market, each engineered to operate in an individual way and often for specific uses or kinds of projects. To help with understanding this still-growing methodology, we have put together the following primer to help explain the common applications and general use of epoxies in old-house restoration as a guide for understanding what’s possible for matching products and projects.

The A and B components of fillers are usually different colors to help with proper proportioning and thorough mixing—critical for complete curing of the epoxy

What is Epoxy?

Epoxy first became a household word in the 1960s, when a new breed of super-strong adhesives started to appear in local hardware stores. Its extensive use dates back as far as the 1940s. Like plywood, Plexiglas, and many other innovations that later became construction materials, epoxy technology was perfected during World War II, in this case as an alternative to metal fasteners for joining members in aircraft production. Briefly, epoxies are petroleum-based resins that cure to a solid state when combined with the right amount of the appropriate hardener. Epoxies are two-part systems—typically cans or tubes labeled A and B—that must always be mixed immediately before application. In contrast, a one-part system like yellow carpenters’ glue is usable right out of the bottle at any time.

Epoxies are also thermosetting—that is, they create heat as they cure. This is important to remember because substantial quantities of mixed resin and hardener—excess left in a mixing container, for example, or a pouring 1″ or more thick—can generate enough heat to melt the container or burn the epoxy. For the same reason, resin that is warm or used in a warm environment will cure at an accelerated rate. (Direct sunlight also will speed up the curing reaction.) The good news is temperature can be used to improve the working properties of the epoxy during adverse conditions, by cooling the resin in an ice bath on hot days to increase the pot life, or circulating warm air around a project in cold weather.

Consolidant effectively saturates the deteriorated wood of thin object, such as window sashes, with repeated brushings that pay particular attention to end grain. Large projects often require small holes for deeper penetration.

Consolidants and Fillers

Epoxies have really come into their own in restoration work for the repair and reconstruction of wood building parts. Where the component still retains its general shape, but has lost much of its integrity due to rot, fungal growth, or insect damage, epoxy consolidants can be used to reinforce the remaining wood fiber. Generally speaking, the consolidant is resin that has been formulated for low-viscosity so that porous wood readily absorbs it like a sponge. Ideally, the consolidant penetrates right to the threshold of sound wood, returning the damaged area to a significant percentage of its former strength and consistency when it cures. In small projects, resin and hardener mix is often simply brushed repeatedly onto the component surface as the wood drinks it up. Larger projects often require perforating all of the damaged wood with small holes in strategic spots to enhance saturation, then filling these holes with consolidant mixture from a squeeze bottle. (Consult the manufacturers’ recommendations for instructions.)

The typical consolidation candidates in old houses are the rotted parts of window sash, doors, and carved or molded features, such as newel posts and column details. Indeed, the beauty of consolidants from a practical (as well as historic preservation) standpoint is that they retain as much as possible of the original wood. Moreover, they can often be used without removing the building part from its installed position. Epoxy consolidants, however, are for the most part not reversible; once they have cured they cannot be undone with, say, a special solvent. Consolidants help restore the integrity of wood, but it takes epoxy fillers to replace lost wood fiber.

These materials are resins and additives combined (often in proprietary mixtures) to make a gap-filling paste. Unlike simple one-part wood putty, epoxy fillers are, again, mixed from A and B components and often formulated not only to adhere well to consolidated wood (which helps increase their bond in tough, outdoor conditions) but flex like wood and even tool like wood with planes, chisels, and sandpaper. Autobody fillers and resins, which appear to be similar, are not wood restoration epoxies. Generally, these are polyester resins that cure with the addition of a catalyst. They are not formulated for absorption by wood. The fillers in particular are engineered to adhere to and move with sheet metal, and therefore they may not stay in place on, say, a wood window sill. Epoxy fillers are ideal for filling the voids and holes in wood: checks and splits from weathering; holes from old hardware, countersunk screws and nails; lost knots and woodpecker holes.

Epoxy fillers bond best to wood that was previously consolidated and cured. Note how the weathered wood has completely absorbed the low-viscosity consolidant applied earlier.

However, the right fillers can also be used to reconstruct totally lost features by building up the filler in several stages, then sculpting or forming it with woodworking tools. The epoxy curing process actually has three stages, and this can be a great advantage when working with fillers. When the liquid resin or paste filler is first mixed with hardener, it remains in an easily manipulated first stage called the open time or working time. Before the epoxy cures to the third solid stage, it passes through a second stage, often called the gel or kick-off stage. In this short stage, the epoxy has begun its initial cure; while it is no longer a liquid it is not quite yet a hard solid. When fillers reach this soft, rubbery stage, the time is ideal to rough out general shapes and remove large amounts of material by quickly sculpting them with “cheese grater” tools (such as Stanley Surform planes). With experience, it’s possible to quickly work the filler without breaking its bond to the wood, waiting until the filler has cured hard for final shaping and sanding.

Important Considerations for Epoxy

Consolidants and fillers are very versatile and user-friendly, but they still require proper preparation and evaluation of their appropriateness before moving ahead with a project.

Moisture: The root of all building problems, moisture is a critical issue with epoxies. Wood damaged by rot or fungal growth needs to have 1) the source of moisture corrected and 2) the wood thoroughly dried through air circulation or heaters. Epoxies cannot penetrate wood fiber that already has a high moisture content such as might be found in the inner sections of a post or column. Moreover, if the consolidant is applied so that it cures on only the dry outer surface of such a member, it will trap the moisture inside the wood where it will continue to promote damage. For this reason, some restoration conservators use consolidants in conjunction with wood preservatives, such as borates, when working on large repairs.

When cured, epoxy fillers formulated for wood restoration can be shaped and sanded the same as hardwoods.

Appearance: If the epoxy repair is in, say, a floor that is due to have a clear varnish, its visual impact is worth noting. Epoxy products made for wood restoration are generally formulated with clear or amber resins and wood-toned fillers. Nonetheless consolidants, like any adhesive, can slightly alter the tone of wood, and fillers do not generally take stain the same as wood.

Cost vs. Value: Volume for volume, epoxy fillers and consolidants are more expensive than most woods, so the value—both economically and philosophically—of making repairs with epoxy versus new wood has to be considered. That is not to say that repairs cannot be a hybrid of epoxies and patches of new wood, but the time and expense of a large epoxy repair on a common, mundane building component—a simple porch railing, for instance—may be better spent on a feature that would be much more difficult or costly to replace in-kind, like a carved column capital.

Safety: Like any active compounds, epoxies should be used with proper care and respect by following the manufacturer’s instructions. Wear gloves and eye protection and avoid getting liquid materials, especially hardeners, on skin. Work with good ventilation.

Epoxy technology has proved so effective for the repair and reconstruction of wood building parts that over the last decade the methods and products have expanded to a wider range of building materials. With concrete now in its second century of wide use, epoxy products are increasingly turned to the repair and construction of features built with this material. Epoxy repair techniques for the repair of load-bearing members, such as heavy timber beams, have grown steadily since experiments in the 1970s. As epoxy technology continues to extend its track record and grow in applications, we are bound to see more.

Epoxy glue helped restore this Palladian window. Deteriorated wood was removed, and new pieces were spliced in to support the upper window panes.

When it comes to adhesives, most homeowners shy away from epoxies in favor of more familiar glues because they find them intimidating. Sure, it takes a little work to mix epoxies properly, but they have plenty of advantages that make the extra effort worth it. Epoxies are waterproof and extremely strong, and they resist most solvents. But most important in terms of restoration work, epoxy glues don’t need clamp pressure to set. Understanding how epoxy glues work, and when to use them, can save you time on restoration projects by allowing in-place repairs of layered architectural elements. With a little practice, epoxies could become common materials in your toolbox.

One Hot Compound

Epoxies are basically two-part polymers or plastics. Many of us have had some experience with the epoxy glues available in the popular double-barreled syringe at hardware stores. While these are convenient, they are more costly and less versatile than epoxies you mix yourself. Most epoxy adhesives can be formulated to work well with traditional materials such as stone and metal. (Other epoxies can also be used as fillers and as consolidants, to build up deteriorated wood surfaces.)

It's important to mix resin and hardener carefully, following the manufacturer's recommendations. Using a scale helps.

Generally speaking, epoxies consist of a resin and a hardener. The exact chemical composition of each isn’t critical to understanding and using this material, but there are a few aspects worth noting. Cured epoxies are generally classified as polyamines. Chemically, they’re different from polyesters, which make up the bulk of automobile body patches and fillers. While these have been promoted for restoration work, they do not perform as well as polyamines in exterior applications on wood.

All epoxies that we buy are derived from families of these two components, resins and hardeners. Manufacturers can control various properties such as curing rate, hardness, strength, and flexibility depending on the combination used. Because the hardener becomes part of the product, it is important that both components be mixed as closely as possible to the manufacturer’s recommended proportions. To understand why, it helps to visualize how the two components work together. Imagine the resin as a jar of marble-sized ball bearings, and the hardener as small magnetic discs the size of watch batteries. When you mix both together and stir, they become a tangled mass of intertwined ball bearings. If you add too many ball bearings or too many magnets, there will be weak spots.

Mix only as much epoxy as you need, both to avoid over-applying and because any extra will end up in the trash can. This is easier said than done; it takes practice to figure out the exact amount needed per project. If you have to err, do it on the side of mixing a little too much so you aren’t stuck trying to combine more at a critical repair moment.

Epoxies are exothermic, meaning they generate heat as they cure. This heat accelerates the curing rate until all of the material has cross-linked. Curing times for epoxies are usually given at 70° F. Since curing is so temperature-dependent, a good rule of thumb is to assume that for every 18° F (10° C) change in temperature, the curing speed will either double or halve. So, if the outside air temperature is between 50° and 55° F, an epoxy designed to cure in 6 hours will need 12. The same epoxy will cure in about 3 hours on an 85° or 90° day. To slow (or accelerate) curing time, select epoxies formulated for warm or cool weather. In the summertime, work in shaded areas and transfer mixed material into wide, shallow containers so heat dissipates more rapidly.

The new wood added to this Palladian window, attached with epoxy, is clearly visible.

To Use, or Not to Use

Epoxies bond so strongly that they are difficult to reverse, so they should always be considered permanent. This makes epoxies great for repairs like inserting Dutchman patches into flooring, or securing gingerbread trim or other fragile architectural components that need bonding. But it also means you should consider the entire assembly before reaching for the mixing bowl. Historic windows, for example, typically need maintenance that requires disassembling one or more joints, so you should only use epoxy on non-jointed window elements.

After combining the resin and hardener in a disposable container, like a paper cup, I find it works best to mix thoroughly for a full 3 minutes before applying. Use a disposable brush, and always apply the epoxy to a completely dry surface. You can assure the proper alignment of parts with something as simple as masking tape, but if the pieces want to slide apart, you can use light clamp pressure to stabilize them until the epoxy has cured. Since epoxy is somewhat gap-filling, small inconsistencies in the joint surfaces are easily bridged. When the epoxy has cured, sand or plane away any excess. If the repair is on an exterior element, it must be protected from sunlight, which will break down epoxy polymers, so brush with an oil-based primer before finish coating. To keep epoxy from oozing onto surfaces outside of the glue joint (like the clamp itself), use strips of plastic wrap or waxed paper as a separator. (Epoxy won’t stick to either one.) To protect the epoxy from rain while it cures, loosely cover the repair with plastic sheeting.