How & When To Use Epoxy Fillers

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If your house has damaged wood, epoxy can be an essential restoration material.

If your house has damaged wood, epoxy can be an essential restoration material. (Photo: Ronald Hudson/Fotolia.com)

Over the past 40 years, epoxy has become somewhat synonymous with architectural conservation. But some people are still reluctant to use epoxies in their own homes—not only are they are expensive (routinely costing more than $100 per gallon), but they’re also a little mysterious.

There’s no reason to fear epoxies. Epoxy filler, in fact, can be a cost- and labor-effective way to fix voids left in woodwork by rot or insect damage. Areas that are difficult to replace, or can’t be addressed easily by complete or partial replacement of the wood, are all good candidates for epoxy repairs. Here, I used epoxy to fill voids in a porch column base deteriorated by carpenter ants, but the repair process is the same regardless of the damage.

When to Use Epoxy

Replace the entire element when…

  • The damage coverage is greater than 50 percent.
  • It is square stock that is readily available and easily replaced.
  • This is your third epoxy fill on the same piece of wood.

Use a dutchman patch when…

  • You’re replacing up to 50 percent of the element.
  • It must be drilled for screws or milled for joinery.
  • The final product must be clear finished.
  • It’s important that you respect the original construction and wood species.

Use epoxy filler when…

  • The element would be difficult to replicate with new wood, such as a molded element that would require sophisticated machining or laborious handwork.
  • The element can’t be easily removed without taking apart other components (example: a pegged window sill).
  • Around 80 to 85 percent of the element is still sound material.

Step 1

Step 1: Removing paint and fixing bowed wood

Step 1

Remove any paint and old caulk from the wood—this will allow you to see the full extent of the damage. In this case, the base had taken on an unexpected bowed shape and needed to be flattened before filling (otherwise, the filler would effectively “freeze” the base into that shape). Using a thin-kerf (3/32" thick) circular saw blade, I cut a several slots in the base, then filled them with wedges of red cedar coated in waterproof glue.

Step 2

Step 2: Coat with epoxy consolidant

Step 2

Next, coat the entire base with an epoxy consolidant. Consolidants—thin or slow-curing epoxies that surround the friable wood fibers with a hardening resin—should be used in conjunction with fillers to provide good adhesion. Let the consolidant cure for several hours before moving on to the next step.

Step 3

Step 3: Dam cracks with a paste epoxy filler

Step 3

Once the consolidant has cured, there’s another step before filling. The voids in this piece are not simple holes—because insects made them, they’re an interconnected network of tunnels. If you poured a filler into the holes in the base, it would likely run out of the cracks and all over the work surface. To prevent this, you first need to dam the cracks with epoxy putty; you can use any of the paste-like epoxy fillers that appear on the market. Once you’ve thoroughly mixed the putty (see How To Mix Epoxy), press it into the cracks and crevices along the upper surface and the edge of the base to seal it against leaking.

Step 4

Step 4: Pour liquid epoxy into the wood

Step 4

Once the dams have cured, it’s time to pour a thin liquid epoxy into the wood. This pourable filler is thin enough to flow into all of the unseen insect passages on this column base; it’s also helpful when filling fissured wood, which you might find on a weathered windowsill, for example.

Pour the liquid epoxy until all of the voids appear to be full. Wait for several minutes, and you’ll see depressions develop, which indicates that the epoxy is penetrating into the wood and it’s time to add a little more. Do this until it seems that no more can be absorbed. When this cures, you may find that minor depressions have developed. These can be filled by pouring more epoxy or using ordinary wood fillers to cover tiny holes and pits.

Step 5

Step 5: Sand, paint, and reinstall

Step 5

Once the epoxy has fully cured (the time is dependent on temperature, but it’s best to leave it undisturbed for at least 24 hours), you can sand, paint, and reinstall the wood. While phenolic microballoons make the epoxy sandable, they are plastic, so machine sanding can cause the paper to clog fairly quickly. Keep plenty on hand and use a lower speed.

How To Mix Epoxy

Paste Filler
Paste-type epoxies require mixing two dry, paste-like components, usually in a 1:1 ratio. Wearing latex gloves, scoop half the needed amount from one container (part A); using your other hand, scoop out an equivalent sized ball from the other (part B). Knead the two scoops together until the color is uniform.

B: Stir for at least three minutes

B

A: Pour the epoxy components together

A

D: Add silica if desired

D

C: Add phenolic microballoons

C

Variable Viscosity Filler
Pourable fillers require a little more patience, but they can be extremely versatile. Mix the components, according to instructions, in a paper cup or thick plastic container [A]. Stir the epoxy with a small wooden or plastic stick for a minimum of three minutes [B]. Then add phenolic microballoons (available from epoxy suppliers) to expand the epoxy without weakening it [C]. You can usually add microballoons in a volume equivalent to the mixed epoxy—for example, up to 5 cubic inches of microballoons for 5 cubic inches of epoxy. This will slightly thicken the epoxy, increase the volume, and make it easier to sand when cured. If desired, you can slowly add silica [D] to bring the epoxy to a paste-like consistency.