Filler shrinks. That one fact explains most failed patches
Every filler and every joint compound loses volume as it cures, and it does not lose it evenly: the surface skins over while the middle is still wet, so a thick lump shrinks into a dish and often cracks outward from the centre. Which is why the instruction that sounds backwards is the correct one — fill slightly proud of the wall, about half a millimetre to a millimetre, and let sanding bring it back. Spread it flush and you are guaranteeing a shallow dent.
The second half of the story is why a patch can be perfectly flat and still obviously there. Sanded filler and original painted plaster do not absorb paint at the same rate. Paint straight onto filler and that area dries to a slightly different sheen and depth — invisible head-on, and unmistakable the moment light rakes across the wall. Primer is the coat that equalises absorption, and it is the single most commonly skipped step in the whole job.
Three shapes of damage, three different methods
- Nail hole, screw hole, old plug hole — up to about 25 mm. Fill it directly. But deal with what is already in the hole first: a plastic plug should be pushed all the way in or cut flush, never yanked out (it takes a disc of plaster with it), and a rusty metal anchor has to come out or be cut flat and sealed with two coats of rust paint, because iron left in the wall keeps rusting and pushes the filler back out.
- A punched hole or a door-handle dent — roughly 25 to 100 mm in plasterboard. Cut the ragged edge into a clean shape, cover it with a self-adhesive mesh patch or a piece of fibreglass mesh about 5 cm larger than the hole, then skim joint compound over it and feather the edges out to nothing. Never bridge a plasterboard hole with expanding foam: it is weak, its volume is unstable, and it sags sooner or later.
- A hole bigger than a bowl — cut it into a clean square, fit a timber batten behind the opening fixed through the sound board with plasterboard screws, cut a new piece of board to match, screw that to the batten, tape all four edges, then build up two or three thin coats. Trying to fill a big void with filler alone is how people end up with a crater.
And the crack, which is its own problem
A crack filled without opening it out will simply re-open along the same line, because you have put material on top of a joint rather than into it. Run a knife along both sides to cut a shallow V, fill that with compound, and press a mesh tape into the wet compound so the repair can move slightly without splitting. One exception matters more than the technique: if a long crack keeps returning, or you have diagonal cracks running from the corners of door and window openings, that is movement in the structure rather than a paint fault, and patching it a fourth time is a waste of a weekend.