The short answer
Overfill slightly, let it dry completely, then sand flat. The visible repairs are the ones that were sanded while still soft or painted before they were dry.
- Brush out the hole and, for anything bigger than a screw hole, undercut the edges slightly so the filler has something to grip.
- Damp the hole with a wet finger on plaster, which stops it sucking the water out of the filler and cracking it.
- Press filler in firmly and leave it standing slightly proud.
- Let it dry fully. Hours, not minutes, and longer for a deep fill.
- Sand flat with fine paper on a block, then spot-prime before painting.
What will ruin it
Skipping the primer is why filled patches show through emulsion as dull spots. Filler is more absorbent than the surrounding paint and drinks the first coat.
Why it works
Filler shrinks slightly as the water leaves it, which is why it is left proud, and it dries from the outside in, which is why a deep fill that feels hard can still be soft underneath.
Gas work requires Gas Safe registration by law and most electrical work is notifiable. Where a tip says to call somebody, that is the advice. Full disclaimer.
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