How do I fix a gap in a joint I have already cut?
Whether to re-cut, pack, patch or accept it — decided by whether the gap is structural or cosmetic, and whether the joint will be finished clear or painted.
- Difficulty
- intermediate
- Time
- 30 min
- Read
- 3 min
Short answer
If the gap stops the shoulders pulling up, it is structural and the joint must be re-cut or packed with wood before gluing. If the joint pulls tight and the gap is only visible, patch it with a sliver of the same species, grain running the same way, glued in and pared flush. Filler is only acceptable under paint.
There are two questions and they have to be answered in order. Is the gap holding the joint open, or is the joint tight and the gap merely showing? A gap at a shoulder line is the first kind and has to be dealt with. A gap at the tip of a dovetail pin is the second kind and is a cosmetic decision.
Step by step
- Assemble dry and see whether the joint pulls up.If the shoulders meet and the gap is only at an edge or a tip, the joint is sound. If the shoulders stand apart, the joint is not doing its job and needs correcting.
- For a structural gap, find what is holding it open.Chalk the mating surfaces, push together, pull apart, and correct only where it rubs. Waste in a corner, a bulge in a cheek and an unsquare shoulder are the three usual culprits.
- Pack a loose joint with wood, not glue.Glue a shaving, a slice of veneer or a thin slip of the same species to one face, let it cure fully, then pare it to fit. A thick glue line is a weak glue line — glue is strongest as a thin film.
- For a cosmetic gap, cut a patch rather than filling it.Split or saw a sliver from the same board, so the colour and the grain match. Taper it so it wedges in, glue it, tap it home proud, and pare it flush when cured.
- Match the grain direction of the patch to its surroundings.A patch with the grain running the wrong way catches the light differently and shows more than the gap did. Orient it the same way as the wood it sits in.
- For very fine gaps, move the fibres instead.Tap the wood beside the gap over with a small punch or a burnisher. In tight-grained hardwood, this closes a hairline invisibly and leaves no foreign material in the joint.
- Use filler only under paint.Two-part or fine-surface filler sands hard and takes paint well. Under a clear finish it absorbs differently from the wood and appears as a pale or dark shape the moment oil or stain goes on.
- Do not use glue and sanding dust under a clear finish.It looks convincing dry and terrible once finished, because the glue seals the wood fibres and stops them taking the finish.
Tips
- Cut patches from an offcut of the same board, kept for exactly this. The colour match from a different board of the same species is never as good.
- A gap you can see from a metre away is worth fixing. One you can only see with your nose against the work usually is not.
- If several joints in a run all have the same gap, fix the setup or technique before repairing them, or you will repeat the repair on the next piece too.
Common mistakes
- Forcing a joint closed with clamps to hide a gap — The wood is now under permanent stress and the joint is doing structural work it was not designed for. It usually opens again, or splits somewhere else.
- Filling a structural gap with epoxy and calling it done — Epoxy will hold, but a joint that could not pull up was probably wrong in more than one dimension, and the piece will be out of square permanently.
Questions people ask
Can I use sawdust and glue as a filler?
Under paint, yes. Under a clear finish, no — it dries a different colour from the wood, seals the surrounding fibres and shows as a halo once the finish goes on.
Is it worth re-cutting the joint?
If the piece is small and the timber is cheap, almost always. Re-cutting takes less time than a good patch and produces a better result. On a large or expensive component, patch it.