Why does my filler keep cracking or sinking?

Filler failure is not really a product problem. Almost every failed repair comes down to one of four things, and each has an obvious signature once you know what to look for.

What you'll need

• Fine surface filler and a deep-gap filler
• Flexible decorator's caulk
• A filling knife and a flexible spreader
• PVA or a plaster sealer
• A damp brush
• Abrasive paper

Step by step

1. Look at how it failed.A dish in the middle means shrinkage from a thick application. A crack along a line means movement. Powdery, crumbly filler means it dried before it set. Filler that fell out means no key.
2. Check the depth you filled.Most fine fillers are designed for a few millimetres. Fill 20mm in one pass and the outside sets while the middle is still shrinking, so it dishes and cracks.
3. Use a deep-gap filler for depth.Lightweight and deep-fill fillers are formulated to go in thick without shrinking. Use one for the bulk and a fine filler for the last 2mm.
4. Build up in layers otherwise.Fill in passes of 5mm or less, letting each set. It is slower in wall-clock time and faster in working time, because you are not redoing it.
5. Identify a moving joint.Wall to ceiling, wall to skirting, wall to architrave, or along a plasterboard joint. These move seasonally and no rigid filler will survive there.
6. Use caulk or a flexible filler on movement.Decorator's caulk on junctions between components, or a flexible acrylic filler on a crack that has reopened before.
7. Check the suction of the wall.Flick water onto the surface. If it vanishes instantly, the wall is very absorbent and will pull the water out of the filler before it hydrates.
8. Damp or seal a thirsty wall.A damp brush over the area just before filling, or a coat of diluted PVA or a plaster sealer, allowed to dry. This alone cures most crumbly repairs on old plaster.
9. Rake out and undercut a crack.Filler pressed into a hairline crack has nothing to hold onto. Open the crack out with the corner of a scraper, undercutting slightly so the filler is keyed in.
10. Fill proud and sand back.All fillers shrink a little. Leaving it slightly proud and sanding flush is quicker than a second pass in a hollow.
11. Sand once it is genuinely hard.Sanding filler that is still soft drags it out of the repair and clogs the paper. Give it the time on the tub.
12. Prime before painting.Filler is more absorbent than the wall around it. Unprimed, it shows through the topcoat as a flat patch even when the repair itself is perfect.

Tips

• Two products cover almost everything: a lightweight deep-fill for volume and a fine surface filler for the last coat. Trying to do both jobs with one is why repairs fail.
• A flexible spreader rather than a rigid filling knife gives a much flatter finish on the final coat and needs far less sanding.
• Mist the wall with water from a plant sprayer before filling old, dusty plaster. It is the single most effective thing you can do for a repair on a thirsty wall.

Common mistakes

• Filling a deep hole in one pass with fine surface filler — Fine fillers set from the outside in. A thick application skins over while the core is still shrinking, so it dishes, cracks and often stays soft in the middle for weeks.
• Using rigid filler on a wall-to-ceiling or wall-to-skirting joint — Those joints move by a millimetre or more with the seasons. Rigid filler cannot accommodate it, so it cracks along its length and the result is more visible than the original gap.

If it doesn't work

Questions people ask