What are the steps for filling cracks in wood?
Do it yourself · Repairing
RepairingFilling cracks in wood
Not every crack should be filled. A crack that is still moving will sooner or later push any filler back out.
- First: is it still moving?
- Outdoors, fill flexibly
- Structural = don't fill
First work out what kind of crack it is
There are three kinds, and only for the first is filling the right response.
A drying check happens because wood shrinks across its width while hardly moving along its length. That stress comes out as a crack along the grain. In solid beams and thick boards it is normal, and once the wood reaches its equilibrium moisture content, the crack stops moving. Then you can fill it.
A moving joint moves with the seasons: open in winter, closed in summer. You don't fill it with a hard filler — that gets squeezed out again in summer or cracks open next to it. This calls for a permanently elastic sealant, or nothing at all.
And then there are cracks that run across the grain, or form around a joint or a knot. Those are not cosmetic cracks but warning signs. In a load-bearing part, filling is then the wrong answer to the wrong question.
Draw a short line across the crack and note the date. Look at it again a few months later. If the two halves of the line have shifted, the crack is still moving and hard filling makes no sense.
- Drying check along the grain
- normal; can be filled once it has stopped moving
- Moving joint
- elastic sealant or nothing
- Crack across the grain
- structural warning sign, don't fill
- Indoors
- wood filler or two-part paste
- Outdoors
- flexible or two-part repair paste
- Large cracks
- fill with a wooden sliver
- Always
- overfill slightly, because filler shrinks
To be fair: a filled crack stays visible. Filler takes stain and oil differently from wood, and after a year the repair shows as a lighter or darker line. If you want an invisible result, you end up with a glued-in wooden sliver of the same wood — more work, but the only thing that really disappears.
How does the crack run?
Pencil test: after a few months
Where?
How wide?
Must the colour match?
Filling a crack, in order
- Assess — is it moving, along or across, is the wood around it hard?
- Clean out — dust, old filler and loose fibres out
- Let it dry — filling damp wood always fails
- Apply — press it in with a filling knife, overfill slightly
- Let it cure — a little longer for thick layers
- Sand flat — with a sanding block, not with your fingers
- Finish — the whole surface; test the colour first
To be fair: filler takes stain and oil differently from wood — after a year the repair shows.
Repairing scratches and dents →Which filler for what
| Product | What for | Watch out |
|---|---|---|
| Water-based wood filler | small cracks and holes indoors | shrinks as it dries: overfill slightly |
| Two-part repair paste | larger holes, outdoors too | cures quickly; mix in small amounts |
| Elastic sealant | moving joints and connections | cannot be sanded; smooth it off with a wet finger |
| Sawdust with wood glue | small cracks where colour matters | use sawdust from the same wood, not sanding dust |
| Wooden sliver | wide drying checks in solid wood | the most work, and the only thing that becomes truly invisible |
| Epoxy | structural repair of decayed wood | work for those who know what they're doing; not just casually into a beam |
Filling a crack
- AssessIs it still moving, does it run along or across the grain, and is the wood around it hard? This decides whether you should fill at all.
- Clean outRemove dust, old filler and loose fibres with a small brush or compressed air. Filler does not stick to dust.
- Let it dryFilling damp wood always fails. Leave outdoor work for at least a few dry days.
- ApplyPress the filler in with a filling knife rather than smearing it over, so the crack is filled from the bottom. Overfill slightly.
- Let it cureFollow the time on the packaging, and add a little for thick layers. Sanding too early gives a torn, rough surface.
- Sand flatWith a sanding block, not with your fingers round it — otherwise you sand a hollow around the repair.
- FinishingFinish the whole surface, not just the spot. Test colour and sheen first on an inconspicuous corner.
Preventing cracks
Let wood acclimatise
Two days in the room where it will stand. Wood you use straight away shrinks afterwards and cracks where it is fixed.
Treat the end grain
End grain dries out faster than the rest, and that is exactly where cracks start. An extra coat on the end grain makes a visible difference.
Avoid full sun on fresh wood
A new table standing in the blazing sun in its first summer dries out much faster on one side than the other — and that pulls it out of shape and cracks it.
Let the wood move
Fix wide parts so that they can shrink: slotted holes instead of fixed screw holes. Whatever cannot move, cracks.
A solid oak or Douglas fir beam almost always develops drying checks along its length. That is not a fault and usually no problem for its load-bearing capacity. Filling is then an aesthetic choice, not a repair.
Frequently asked questions about cracks in wood
Do I need to fill every crack?
No. Drying checks along the grain in a beam are part of it and usually don't affect its strength. Filling is then purely cosmetic.
How do I know whether a crack is still moving?
Draw a short line across the crack, note the date and look at it again months later. Shifted halves mean: still moving.
Which product do I use outdoors?
A two-part repair paste for larger holes, or a permanently elastic sealant for joints that move. Ordinary water-based wood filler doesn't hold up outdoors.
How do I make the repair invisible?
With sawdust from the same wood mixed with glue, or with a glued-in wooden sliver. Filler always takes stain differently from the surrounding wood.
Can I fill and then stain?
Yes, but test first in an inconspicuous place. Most fillers take stain lighter or darker than the surrounding wood.
When should I get someone to look at it?
With cracks across the grain, with cracks around a joint in a load-bearing part, and if the wood around it feels soft.
Other repairs
Scratches, dents and water damage each call for a different approach.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Carpinus betulus — Beentree, CC BY-SA 3.0 · Wide crack in a weathered decking board outdoors — Scott Basford, CC BY-SA 3.0