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.
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.
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.
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 (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.
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.
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 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 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.
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.