Two-part wood repair epoxy holds where ordinary filler falls out. It bonds into the timber and cures hard enough to sand, drill and paint, so a repaired sill or rail behaves like solid wood again. It is not hard to use, but a few habits separate a repair that lasts decades from one that lifts within a year. This guide covers mixing, working time, shaping and the mistakes we see most often.
Where to see it done
We are filming a full walkthrough and will add the video here once it is ready. In the meantime, our rotted window sill repair guide shows this exact epoxy used on a real sill, with photos of the soft timber being taken back to sound wood and new timber spliced in before the epoxy goes on. That is the clearest picture of where two-part epoxy fits into a proper repair.
Mixing ratios without guesswork
Two-part wood repair epoxy comes as a resin and a hardener that only work once they are combined in the right proportion. Always mix to the ratio printed on the pack. If you guess, you get soft spots that never fully cure and stay tacky under the paint.
Two things make the biggest difference:
- Mix until the colour is completely even. Streaks and swirls mean the hardener has not reached all of the resin. Keep folding and scraping the sides of your mixing board until there is one uniform colour with no marbling.
- Mix only what you can use in time. A big batch generates its own heat and goes off faster in the tub than it does spread thin on the timber. Small, frequent batches waste less.
Use a clean offcut or an old tile as a mixing board and a stiff filling knife to combine it. If you are doing more than the occasional repair, a heavy duty epoxy gun dispenses cartridge epoxy through a mixing nozzle, so the ratio comes out right every time and your hands stay a lot cleaner. It is the upgrade worth making once you are filling more than one window.
Working time by temperature
The working time, sometimes called pot life, is how long you have to spread and shape the epoxy before it stiffens. Temperature changes it more than anything else. On a warm day the epoxy goes off noticeably faster, and in the cold it slows right down.
That is why cure speeds are worth matching to the job:
- The DT one hour cure epoxy sets fast, which suits small fills and cooler working conditions where you want to move on quickly.
- A slower four hour cure gives you more time on the board and on the timber, which helps when you are building up a large area or working in the heat.
Epoxy also needs a bit of warmth to cure properly. If you are working on a cold day, bring the sash somewhere warmer to set, or wait for the milder part of the day. Do not try to rush a cold cure with a heat gun held close, because that can skin the surface while the inside stays soft.
Shaping before cure versus sanding after
There are two ways to get the final shape, and the best results usually come from using both.
Shaping while it is soft
While the epoxy is still workable you can press and draw it to roughly the right profile with a filling knife. Getting close to shape now means far less sanding later. Leave it slightly proud of the surrounding timber rather than trying to feather it perfectly, because you will refine it once it is hard.
Sanding once it has cured
After it cures fully, the epoxy sands, carves and planes much like timber, so this is where you get the precise finish. Work from a coarser abrasive down to a finer one and check the profile against the sound timber next to it as you go.
If you are sanding old window paint at the same time, treat it as though it contains lead. Wear gloves and a dust mask, keep the dust down rather than letting it fly, and wash your hands before you eat, drink or smoke.
Common mistakes
- Filling over rotten timber. Epoxy is not a cure for active rot. Remove the soft, punky wood back to sound timber first, then rebuild. Our sill repair guide shows how far to go.
- Applying to damp or dusty wood. The bond depends on clean, dry, sound timber. Brush the dust out and let wet timber dry before you start.
- Under-mixing. This is the most common reason a repair stays soft. When in doubt, mix for longer.
- Reaching for the wrong product. Epoxy earns its place on structural repairs and areas that take weather and movement. For small cosmetic dents a filler may be all you need. Our comparison of epoxy versus builders bog and filler walks through which suits which job.
- Leaving it bare outside. Once cured and sanded, prime and paint the repair. Paint keeps it matching the rest of the window and protects the finish over the years.
FAQ
How do I get the mixing ratio right?
Mix to the ratio printed on the pack and keep going until the colour is one even shade with no streaks. Streaks mean the hardener has not reached all of the resin, which leaves soft spots. A cartridge and mixing nozzle in an epoxy gun doses the ratio for you if you would rather not measure by hand.
How long do I have to work with it before it sets?
It depends on the cure speed and the temperature. A one hour cure gives you less time than a four hour cure, and both go off faster on a warm day and slower in the cold. Mix small batches so you are never racing a tub that is stiffening.
Can I sand and paint two-part epoxy?
Yes. Once it has fully cured it sands, carves and planes much like timber, and it takes primer and paint well. Shape it roughly while soft, then sand it to a precise profile once hard.
Will it stick to rotten wood?
No. It needs clean, dry, sound timber to bond to. Cut the rot back to solid wood first, then rebuild with epoxy.
Do I need the epoxy gun?
Not for the odd small repair, which you can mix by hand. If you are working through several windows or want the ratio handled automatically and less mess, a heavy duty epoxy gun makes the job faster and cleaner.