How do I fill and sand a wall properly before painting?

The finish you get is decided almost entirely at this stage, and the two mistakes that ruin it are filling in one thick pass and sanding while impatient.

Prepare the hole first. Scrape out any loose material and undercut the edges slightly so the filler has something to grip. Brush out dust — filler will not bond to it. For anything porous or crumbly, damp the area very lightly with a sponge; dry plaster sucks moisture out of filler and causes it to shrink and crack.

Fill proud, in thin layers. Press filler firmly in with a flexible filling knife, working across the hole rather than along it, and leave it standing slightly above the surface — nearly all fillers shrink as they dry. For holes deeper than about 5mm, build up in two or more layers, letting each dry, rather than one deep fill that cracks and sinks.

Let it dry fully. Sanding a filler that is still damp underneath drags it out of the hole and clogs the abrasive. Deep fills may need overnight.

Sand in stages — around 120 grit to knock back the bulk, then 180–240 to finish. Use a sanding block, never bare fingers, which round off the repair into a shallow dip. Feather the edges out into the surrounding wall so there is no ridge.

Check with raking light. Hold a torch or lamp almost flat against the wall. Every flaw you missed appears instantly — and every flaw the paint will later reveal.

Prime the repair. Filler is far more absorbent than the surrounding paint, so an unprimed patch shows through as a dull spot, a phenomenon called flashing. A mist coat or a spot of primer prevents it.