Fill and sand before painting

Find it before you fill it

Damage hides in flat light. Hold a lamp close to the wall so the beam rakes across the surface at a shallow angle and walk it along. Every dent, ridge and old fixing throws a shadow that is invisible from the middle of the room under a ceiling light.

Mark what you find with a pencil dot rather than trusting memory. A wall with fourteen small defects will lose four of them between finding and filling.

Cut back to something sound

Press a filling knife gently around each defect. Anything that lifts, crumbles or sounds hollow is not going to hold filler, and a repair made on top of it will crack along exactly the line you tried to hide.

Rake out a crack slightly rather than skimming over it. A crack with parallel sides holds filler far better than a hairline that closes at the surface. The aim is a small trough with clean, firm edges.

Brush the dust out of the recess before filling. Filler bonds to the wall, not to the dust sitting on it.

Fill proud, in layers

Load the knife and press the filler into the recess rather than wiping it across the opening. Pressing forces out the air; wiping bridges it. Then draw the knife across at a low angle to leave the filler standing slightly above the surface.

Deep damage takes two goes. A thick single fill dries at the surface first, traps moisture behind the crust and sinks later, sometimes weeks later. Half fill it, let it harden, then fill the rest.

Leave the filler alone until it has properly hardened. Working it while it is still soft drags it out of the hole and smears it across sound wall, which then has to be sanded too.

Sand level, in the right order

Use the grit ladder in sequence. Eighty grit knocks down anything standing well proud and is aggressive enough to scar the wall around it, so keep it on the filler itself. One hundred and twenty grit levels the repair into the surrounding surface. One hundred and eighty grit takes the scratches out of the one hundred and twenty.

Keep the abrasive on a block. A block spans the repair and cuts the high points; fingers press into the low points and cut those instead, which is how a flat repair becomes a shallow bowl.

Check with the raking light again, and check with the back of a hand. Skin finds a ridge that an eye will miss, particularly on a wall that is going to end up in a satin or silk finish where every ripple catches the light.

Dust off properly

Vacuum the wall, then the floor, then the wall again. Filler dust is fine enough to hang in the air for a long while and settle back onto the surface after you have wiped it.

Wipe the sanded areas with a barely damp cloth and let them dry. A trace of dust under paint is what makes a finished wall feel like fine sandpaper, and the only cure for that is to sand and paint again.

Prime bare filler before the first full coat if the filler is noticeably more absorbent than the wall around it. Otherwise the repairs drink the paint and read as flat patches through the finish.