What is two-part filler and when do I actually need it?

Two-part filler is the same class of material as car body filler, and that is where most people meet it. It solves a specific set of problems that ordinary filler cannot: repairs that need real strength, repairs that must not shrink at all, repairs that will be shaped or drilled, and repairs outdoors. It is over-specified for filling a nail hole and it is the only sensible answer for rebuilding a rotten window cill.

Safety

What you'll need

• two-part filler and its hardener
• a non-absorbent mixing board and a flexible spreader
• nitrile gloves and eye protection
• abrasive paper, 80 through to 240 grit
• a hardener or preservative treatment, for timber repairs
• a dust mask for sanding

Step by step

1. Decide whether the repair needs it.It earns its place where the repair will be drilled or screwed into, shaped with a chisel or plane, loaded, or exposed to weather. For a hole in a plaster wall that will simply be painted, it is unnecessary, harder to sand and more expensive.
2. Prepare the substrate as if you were gluing.It bonds rather than merely filling, so the surface must be sound, clean, dry and keyed. On timber, take the repair back to sound wood; on metal, abrade to bright metal and degrease. It will not stick to rot, rust or dust.
3. Treat rotten timber before filling it.Cut out everything soft, then treat the remaining timber with a wood hardener and a preservative and let it cure. Filling over rot leaves the decay active behind a waterproof cap, which accelerates it.
4. Mix small quantities and get the ratio right.Usually a small bead of hardener to a golf-ball of filler, and the packet gives the proportion. Too much hardener sets it before you can use it and can leave it brittle; too little leaves it permanently soft and sticky.
5. Mix by folding, not stirring vigorously.Fold the two together on a flat non-absorbent board with a spreader until the colour is completely uniform with no streaks. Whipping air into it leaves pinholes that show up when you sand.
6. Apply it within the working time and press it in hard.You usually have three to five minutes. Press it firmly into the repair so there are no voids behind, and build it slightly proud of the finished shape. It does not slump, so it will hold a proud profile.
7. Shape it at the cheese-hard stage.There is a window a few minutes after it sets where it cuts easily with a surform, a rasp or a sharp chisel. Doing the bulk shaping then saves a great deal of sanding — once it is fully hard it is very tough.
8. Sand through the grits and check the profile.80 grit to shape, then finer to finish. Sand across the repair rather than along it so you do not dish the middle. Fine surface filler over the top deals with any pinholes.
9. Prime before painting.It is non-porous and dense, so paint behaves differently on it than on the surrounding material. A primer evens out the absorption so the repair does not flash through the finish.

Tips

• It sets far faster in warm weather. On a hot day, mix smaller batches and expect half the working time.
• It reaches its full hardness quickly, so there is no reason to leave it overnight before sanding — an hour is usually plenty.
• Wood-specific two-part fillers are formulated to flex slightly with the timber and to take stain a little better than the automotive kind. On visible joinery they are worth the difference.
• A large deep repair should be built in two or three applications rather than one large mass, both to control the heat and to keep the shape.
• Keep the hardener tube capped and out of the sun. It degrades and the filler then sets slowly or not at all.

Common mistakes

• Guessing the hardener quantity — Too little leaves it permanently tacky and it never sands; too much makes it go off on the board and can leave a brittle repair. Follow the ratio on the packet.
• Filling over rot — The decay is still active and now sealed under a waterproof cap. The repair falls out in a year with the surrounding timber having got worse, not better.
• Mixing more than you can apply — It sets on the board and is wasted, and a large mass gets hot. Mix a handful at a time.
• Using it as a general wall filler — It is much harder to sand than ordinary filler and it is dense, so it flashes through paint. It is the wrong tool for a nail hole.

Questions people ask