Search for “rotten wood repair epoxy” and most results show the same move: dig out the soft wood, mix some epoxy, fill the hole. That works for one kind of rot and fails for others, because epoxy does two different jobs. A thin penetrating epoxy soaks into soft wood and hardens it from the inside. A thickened epoxy paste rebuilds the shape that rot took away. Which one you need, and in what order, depends on how deep the rot goes and how wet the wood still is.
This guide sorts the choice by rot depth, then walks the steps using the West System Wood Rot Repair Epoxy instruction sheet (June 2026), the System Three RotFix data sheet, a This Old House walkthrough and the National Research Council of Canada’s digest on wood decay. Where they disagree, the post says so.
The short answer: match the system to the rot
| What you see | What it needs | Why |
|---|---|---|
| Soft, spongy surface, shape still there, sound wood underneath | Penetrating epoxy only | It soaks in and reconsolidates the fibers, then you paint |
| Chunks or corners missing, voids a half inch deep or less | Penetrating epoxy first, then thickened epoxy | The neat epoxy has to soak in before the paste goes on top |
| Voids deeper than about 12 mm (roughly 1/2 inch) | Thin layers or a wood block set in the epoxy | Thick epoxy overheats as it cures |
| Rot that runs through a post, sill or beam, or the wood stays wet | Fix the water, then replace or call a pro | Epoxy is a repair for trim and sills, not a structural cure |
Step 1: find the edge of the rot, and stop the water
Epoxy does not kill rot. System Three’s RotFix data sheet says so directly: it “does not kill rot fungus or keep it from returning,” and points to separate rot-protection products for that. This Old House’s walkthrough has its carpenter add a borate compound to the cavity before the epoxy, which he calls an insurance policy.
The better insurance is removing the water. The National Research Council of Canada’s Canadian Building Digest on decay of wood says decay fungi need roughly 35 to 50 percent moisture content to thrive, with growth best between 65 and 95 degrees F. It also notes that wood is not considered immune until it dries below about 20 percent. A sill that rots again after a perfect epoxy job is a sill that is still getting wet: a failed caulk joint, a missing drip edge, a clogged gutter. Fix that first, and see the exterior window caulk guide if the leak is at the window.
Then cut back to solid wood. Scrape off paint, chisel or grind out anything that crumbles, and probe the edges with a screwdriver. If the rot keeps going the deeper you dig, you are in the bottom row of the table above.
Step 2: dry the wood, and know which number you are chasing
Every source agrees epoxy bonds badly to wet wood. They do not agree on how dry.
| Source | Moisture guidance |
|---|---|
| West System instructions | As dry as possible. Dimensionally stable dry wood usually reads 8 to 12 percent. The epoxy still cures on damp wood, but that reduces adhesion and “may cause continued deterioration” |
| This Old House walkthrough | Below 18 percent on a moisture meter before proceeding |
| NRC Canada decay digest | Wood is not considered immune to fungi until below about 20 percent. Fiber saturation is commonly reached at about 25 to 30 percent |
| System Three RotFix data sheet | No moisture limit stated |
Read that as a ladder. About 20 percent is where decay stops, 18 percent is the outer limit a working carpenter uses, and 12 percent or less is what the manufacturer of the deepest-penetrating product asks for. If you can wait, aim for the lowest. West System suggests a heat gun to speed drying, taking care not to overheat the area, and a moisture meter to check. A moisture meter is the only way to know which rung of that ladder you are on.
Step 3: soak it with penetrating epoxy
This is the step most guides skip. West System describes its epoxy as low viscosity with a slow cure, for deep absorption into deteriorated wood. The sheet’s preparation advice:
- Surface rot: score the wood with a utility knife along the grain to open paths for the epoxy.
- Deep rot or limited access: drill 5 to 6 mm holes in a staggered pattern 25 to 50 mm apart. On a vertical surface, drill them at a downward 45 degree angle so the epoxy does not run out while it soaks in.
- Warm the wood with a heat gun or lamp just before applying. The sheet says this helps draw the epoxy deeper as the wood cools.
- Apply liberally and keep reapplying as the wood drinks it. The wood is saturated when it stops absorbing and a film of epoxy stays on the surface.
Temperature matters more than people expect. At the sheet’s reference 22 degrees C (72 degrees F), working time is 30 to 40 minutes and the epoxy reaches a sandable solid after 24 hours, with full cure in 2 weeks. It is designed for use down to 10 degrees C (50 degrees F). Cooler means thicker, slower and shallower penetration. System Three’s RotFix data sheet gives a minimum application temperature of 45 degrees F, a 30 minute working time at 77 degrees F (100 gram batch) and a Shore D hardness of 75 after a 24 hour cure.
Step 4: rebuild the shape with thickened epoxy
Penetrating epoxy fixes the wood that is left. To replace wood that is gone, you thicken it. West System’s sheet says to apply several neat coats first, then stir in a filler (407 Low-Density for easy sanding, or 405 Filleting Blend for a wood tone) until it reaches a peanut butter consistency that clings to vertical surfaces. The fillers are sold separately from the kit.
Two rules from the sheet keep this from going wrong:
- 12 mm per application. The epoxy is not a casting resin, and curing epoxy makes heat. For a deeper void, build it in thinner layers and wait 3 hours between pours, or set a shaped block of wood (a dutchman) in the void so less epoxy sits in one mass.
- 300 mL per batch, maximum. A cup of that size can generate enough heat to burn skin or melt plastic.
The This Old House approach is a little different: a bonding agent brushed on first, then a two-part epoxy filler from a double-barrel gun, with about 30 minutes of working time and an overnight cure before sanding. Both are the same idea, a consolidated surface and then a paste, from two different product lines.
Step 5: finish it so it survives the sun
Cured epoxy forms a waxy, water-soluble film called amine blush. West System says to remove it with water and an abrasive pad, no soap and no solvents, and then dry the surface with clean paper towel. The sheet also says epoxy is not UV stable, so it needs a UV-stable topcoat, and that it works with most varnishes, polyurethanes and paints. Two details to plan for: one-part polyurethanes and polyester gelcoat can be affected by epoxy amines and should go on only after about two weeks at room temperature, and two-part polyurethanes tend to work best over epoxy. Build a small test panel if you are unsure.
A worked example: a rotted window sill
Suppose a sill has a soft strip 4 inches wide and 24 inches long, 96 square inches of surface, with one corner missing a block about 2 by 4 inches and 1 inch deep.
- Penetrating coverage. West System’s sheet says 100 mL of mixed epoxy covers at least 320 square cm (about 50 square inches), and Epoxyworks gives roughly 4 fluid ounces (118 mL) for about 50 square inches. For 96 square inches that is about 190 to 230 mL, and more if the wood is badly degraded. Mix it as two batches of roughly 100 to 115 mL, comfortably under the 300 mL cap and each used inside the 30 to 40 minute window. The small 12 ounce kit (8 oz resin plus 4 oz hardener, per the manufacturer) makes about 355 mL, so it covers this job once with a little to spare.
- The corner void. 1 inch is 25 mm, and the limit is 12 mm per application, so it takes three layers of thickened epoxy. With 3 hours between pours, that is about 6 hours of waiting, then 24 hours before sanding. A dutchman cuts that sharply: shape a block to leave less than a 12 mm gap, coat the void and block with thickened epoxy, and press it in.
- Volume check. The void is 8 cubic inches, about 131 mL. That is well under 300 mL, which shows the 12 mm depth rule, not volume, is what controls how you build it.
Three products, and what the sources say about each
Abatron Wood Restoration Kit (LiquidWood and WoodEpox, 24 oz). This is a two-product system: LiquidWood is the penetrating consolidant and WoodEpox is the hand-moldable filler. The Amazon listing says LiquidWood has a 30 to 45 minute pot life and the treated wood hardens in a few hours, and that WoodEpox can be mixed and applied by hand to wet LiquidWood with a 20 to 30 minute pot life. Retailer pages describe a 1:1 mix. I could not open Abatron’s own data sheet, so those figures come from the listing, not the maker, and the “no shrinking or cracking” line is the listing’s claim. The kit was in stock when checked, sold by MaxWarehouse, 4.5 stars from about 1,400 ratings with 5 percent at one star, and is marked non-returnable. Reviewers like it for filling rot voids and for sanding and painting, and some find it stiff and slow to dry.
Check price on Amazon: Abatron Wood Restoration Kit. This is an affiliate link (see our disclosure for details).
System Three RotFix (24 oz kit). A penetrating sealer and consolidant, with System Three’s SculpWood putty as the rebuilding step. From the data sheet: mix 100 parts resin to 50 parts hardener by volume (100:43 by weight), 30 minutes of working time at 77 degrees F, minimum application temperature 45 degrees F. Some retailer pages list a 35 degree F minimum and a 72 hour full cure, which do not match the data sheet, so use the data sheet. It does not kill rot, as covered above. In stock when checked, sold by Hartville Hardware, 4.5 stars from about 350 ratings, 3 percent at one star.
Check price on Amazon: System Three RotFix. This is an affiliate link (see our disclosure for details).
West System Wood Rot Repair Kit. The sheet quoted throughout this guide is the most detailed of the three data sets I could read in full, with mix ratio (2:1 by volume), depth, temperature, drilling and finishing guidance. I have not linked it: the Amazon listing for the medium kit showed only 11 ratings when checked, and I would rather not send you to a thin listing. West System’s own site has a dealer list if you want that kit.
When not to repair it with epoxy
- The wood is structural: a post, a sill plate or a beam where the rot runs through the middle. This Old House’s own walkthrough says extensive rot that affects structural integrity may need a professional evaluation.
- The wood is still wet and you cannot find or fix the source. A sealed, wet repair can keep rotting behind the epoxy. West System warns of exactly this on damp wood.
- Replacement is easy and cheap. A short length of trim often costs less in time than a day of epoxy work.
Quick rules that save a failed repair
- Fix the water source first. Epoxy does not kill rot.
- Cut back to solid wood and dry it. Under 12 percent is West System’s target, 18 percent is the outer limit a carpenter used, and about 20 percent is where decay stops.
- Soak before you fill: neat penetrating epoxy first, thickened epoxy second.
- Keep each application to 12 mm or less, and batches to 300 mL or less.
- Wash off the amine blush with water, then use a UV-stable topcoat.
If the damage is a surface crack or small dent rather than rot, the wood filler guide covers the three filler types, and the wood hardener guide covers the thinner, lower-cost consolidants. For choosing between epoxy and other adhesives on a wood joint, see construction adhesive vs epoxy vs silicone. If the rot is around a window and you are weighing DIY against a quote, see what wood rot repair around a window costs, sorted by how far the rot has traveled.
Etienne Basson maintains and repairs his own home, and writes practical guides on choosing the right materials and products for repair and finishing jobs: fillers, sealants, adhesives, and similar. He focuses on what actually works for a specific job, not just a spec sheet.