How to Fill and Sand a Wall So the Repair Disappears

Filling a hole is easy. Making the repair invisible under paint is where it goes wrong, and it goes wrong in predictable ways.

Choose the right filler

Lightweight filler for small holes up to about 10mm. Sands very easily, shrinks a little, not strong.

Powder filler mixed with water for anything deeper. Stronger, cheaper, shrinks less, but you must mix only what you can use in twenty minutes.

Flexible or decorator's caulk for cracks at junctions — where a wall meets a ceiling or skirting. These move seasonally, and rigid filler cracks again within a year. Caulk flexes.

Bonding or one-coat plaster for holes larger than a fist. Filler is not designed for volume and will crack and fall out.

The method

1. Prepare the hole. Scrape out anything loose, and undercut the edges slightly if you can so the filler has something to key into. Brush the dust out.
2. Damp the area with a wet brush for powder fillers. Dry plaster sucks the water out of the mix and causes it to crack as it cures.
3. Press filler in, do not smear it across. Load the knife, push in at an angle, and work it into the depth of the hole.
4. Fill deep holes in two passes. Anything over about 10mm deep should be filled halfway, allowed to set, then topped. A single deep fill shrinks in the middle and cracks.
5. Leave it very slightly proud — under a millimetre. Filler shrinks as it dries and you can always sand off, but a hollow needs a second application.
6. Let it dry fully. Read the tin, then add half again. Sanding damp filler clogs the paper and drags the surface.

Sanding

This is where most repairs are ruined.

• Use a sanding block, not your fingers. Fingers apply pressure in a small circle and dish the middle out.
• Sand in straight strokes, not circles. Circular scratches catch light differently to the surrounding wall and appear under emulsion.
• Feather the edges. Extend your sanding well beyond the filled area so there is no step where the filler meets the wall. A hard edge is the single most common cause of a visible patch.
• Check with a raking light. Hold a torch flat against the wall and look across the surface. Every high spot and hollow becomes obvious. This takes ten seconds and is the difference between a good job and an obvious one.
• 120 grit first, then 180. Skipping to fine paper takes forever; stopping at coarse leaves scratches.

Prime, always

Filler is far more absorbent than the surrounding paint. If you paint straight over it, the filler drinks the emulsion and dries to a different sheen, so the repair shows as a dull patch even though the colour matches perfectly.

Spot-prime every filled area with a coat of the same emulsion thinned with about twenty percent water, or a proper primer sealer. Let it dry, then paint the wall normally.

This one step is the difference between a repair you can find and one you cannot.