Patching a plaster wall before paint

A patch is invisible when the edges are feathered wider than feels sensible, and obvious when the filler stands proud by the thickness of a fingernail.

The temptation with a cracked wall is to load the filler in and move on to the colour. Almost every patch that later shows through a finished coat failed at one of two points: the hole was not opened up enough, or the surrounding paint was still loose when the filler went on.

Opening up sounds destructive and is not. Run the corner of a scraper along the crack until it stops producing dust and starts producing nothing. Anything that comes away with light pressure would have come away later, under the roller, with wet paint attached to it. Undercut the edges slightly so the repair is wider at the back than at the face, which gives the filler something to key against instead of a smooth-sided slot it can shrink away from.

Dust is the other quiet enemy. A brush leaves a fine film in the crack that acts as a release agent. A damp sponge, wrung out until it is barely cool to the touch, takes that film off and leaves the plaster very slightly damp, which slows the filler's set and reduces shrinkage cracking across the middle of the repair.

Fill in two passes rather than one. The first pass is pushed in hard with the knife held almost flat, worked across the crack rather than along it, so the material is driven into the undercut. Leave it a little below the surface. The second pass, once the first has firmed up, is the one that matters visually, and it should be spread from the centre outwards in long strokes that run off the edge of the repair and onto sound paint.

Sanding is where most of the time goes and where most patches are ruined. A sanding block keeps the abrasive flat; fingers behind paper follow the hollow and dish the middle out. Work in circles first to knock the ridges off, then in long straight passes in one direction to remove the circular scratches. Stop when a hand run across the wall with eyes shut finds nothing.

Raking light is the test. A lamp set on the floor and aimed up the wall shows every ridge and hollow that overhead lighting hides. It is an unwelcome sight the first time and it is far better seen before the first coat than after the second.

Prime the patch on its own before priming the wall. Bare filler drinks paint at a different rate from surrounding paintwork, and an unprimed repair reads through even a well applied topcoat as a dull shape where everything else has a slight sheen. A thinned coat of the same paint, cut in over the repair and feathered out, does the job as well as a dedicated primer on small work.

The whole sequence takes an evening, most of which is waiting. A wall patched in one hurried pass takes the same evening and shows the repair for as long as the paint lasts.