Nobody enjoys this part, and it is the part that decides everything. Paint does not fix a wall; it copies it. Whatever is under the film — dust, damp, a loose old coat, a hairline crack — is still there afterwards, only now it is your problem in colour.
Start with what is already on it
Run the flat of your hand across the wall. If it comes away chalky, the old coat is breaking down and has to be washed off and sealed before anything new goes on. Painting over chalk is painting onto dust.
Scrape back anything flaking or blistering until you reach paint that is firmly stuck. Feather the edges with sandpaper so the repair does not show as a ridge through the new coat.
Wash it
Kitchen walls carry grease, and grease is why a coat lifts in sheets six months later. Sugar soap or a mild detergent, then clean water to rinse, then let it dry properly.
Outside, wash off the green and black growth that builds up on a shaded wall through the rains, and let the wall dry for a couple of days before you paint.
Fill and sand
Fill nail holes and cracks proud of the surface, because filler shrinks as it dries. Sand back flush once it is hard.
Hairline cracks that keep coming back are usually movement rather than a bad fill. On an exterior wall a heavy-bodied textured coating will bridge them; inside, a flexible filler and a matt finish is the honest answer.
Prime what needs priming
New plaster, new blockwork, filler patches and bare wood all drink paint. Without a primer the first coat sinks in unevenly and dries patchy, and you spend a whole extra tin of topcoat fixing it.
A primer sealer costs less per litre than a finish coat and does the job in one pass. It is the cheapest step here and the one most often skipped.
Mask last, and take it off early
Tape the frames, the sockets and the skirting after the dust has settled, not before.
Pull the tape while the last coat is still slightly soft. Left until the paint is fully hard, tape takes a strip of the new finish with it.
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