It's also entirely invisible in the finished result, which is exactly why it's the first thing to disappear from a cheap quote.
What preparation actually involves
Protection. Floors covered, furniture moved and sheeted, fittings and sockets masked. Ten minutes here saves an hour of cleaning later.
Cleaning. Walls washed down where there's grease, nicotine or general grime — kitchens and hallways in particular. Paint doesn't bond to grease.
Raking out cracks. A crack has to be opened up before it's filled. Smearing filler over a hairline crack leaves it bridged rather than filled, and it reopens within months. Rake it out, fill it properly, and it stays shut.
Filling. Nail holes, chips, gouges and old fixings. Filler shrinks slightly as it dries, so anything deep needs building up in layers rather than one thick application.
Sanding. Filled areas flattened back level. Also flattening previous patches — old touch-ups sit slightly proud and catch the light, which is why patchy walls look patchy even after repainting.
Sealing. New plaster, filled areas and any bare patches sealed before the finish coats. Skip this and the porous areas absorb more paint than the surrounding wall, leaving a visible difference in sheen — the flat, dull patches people notice a week later and can't explain.
Stain blocking. Water marks, smoke damage, felt-tip and old damp staining need a stain blocker. Emulsion will not cover them, however many coats go on. They bleed straight back through.
New plaster
Fresh plaster needs to dry fully — usually several weeks, longer in winter — and then a mist coat: emulsion thinned with water, applied first so it soaks in and gives the finish coats something to grip.
Painting new plaster with full-strength emulsion and no mist coat is one of the most common DIY mistakes. The paint sits on the surface, and later peels off in sheets.
Preparing woodwork
Doors, frames and skirting need rubbing down to give a key, any bare wood primed, and knots sealed with a knotting solution. An unsealed knot will bleed resin through a white finish within a year — and it's guaranteed on new pine.
Order of work
Ceilings, then walls, then woodwork last. Doing it in that order means any splashes land on surfaces not yet finished.
Exterior preparation
Same principles, higher stakes:
- Full wash-down, and algae treatment where there's green or black growth
- Render repaired — cracks opened, filled and made good; hollow or loose render cut out
- Stabilising primer on any chalking surface. Non-negotiable. Without it the new coat bonds to dust
- Metalwork prepared separately — see painting metal gates, grilles and railings
How to spot a quote that's skipped it
If a quote doesn't mention preparation at all, it isn't priced in. Two quotes with a large gap between them are rarely quoting the same job — one has usually assumed a wash and two coats.