Can I just paint over a damp patch?

No. Paint will not cure damp or mould, and it fails over both. Find and fix the cause first, treat any mould, let the wall dry, then paint.

Preparing walls before painting

Paint holds on a clean, sound, dry surface and fails on a dirty, flaking or damp one. What you need for preparation depends on what the walls are like now, so look at them before you buy anything.

The basic list for a sound, previously painted room

Sugar soap or wall cleaner. Paint grips a clean surface. One container does a room.
Filler for cracks and screw holes. A small tub or a tube is usual for a sound room.
Decorator’s caulk for the gaps where walls meet skirting, architraves and ceiling. How many tubes depends on the size of the gaps; the tube says how far it goes.
Abrasive paper or sanding sponges for rubbing down filler and keying the surface.
Masking tape, commonly in 25 m or 50 m rolls, or a cutting-in brush and a steady hand instead.
Dust sheets for the floor and the furniture.
Tools: a roller frame with a sleeve for each paint, a tray, a brush for cutting in, an extension pole for the ceiling, and a filling knife and scraper.
A dust mask, eye protection and gloves. Sanding makes dust; cleaners and fillers are best kept off skin and out of eyes.

A sound, previously painted matt surface usually needs no primer. Check the paint’s instructions for a strong colour change or a glossy surface.

Six answers that change the list

BuildBatch asks one question about the walls. Each answer changes the preparation lines.

Previously painted and sound. The list above.
Bare or new plaster. Bare plaster soaks up paint. It needs a mist coat (emulsion thinned as the tin says) or a primer-sealer before the top coats, and two top coats straight onto bare plaster may not cover. The paint’s instructions decide the dilution or the primer, and whether two top coats are enough after it.
Stains showing through. Find the cause first. Then a stain-blocking primer on the affected patches; how much depends on the size of the patches and the primer’s own coverage figure.
Damp or mould. Paint will not cure either. The cause has to be found and fixed, and the mould treated, before any paint goes on. Mould treatments carry their own safety instructions: ventilate, wear gloves and eye protection, keep children and pets away. If the cause is not obvious, get advice.
Flaking, powdery or glossy paint. Flaking or powdery paint comes off back to a sound edge, and the surface is filled and sanded. Glossy paint is keyed or primed so that emulsion grips. Expect more filler, more abrasive and possibly an adhesion primer; the paint’s instructions say which.
Wallpaper, textured coatings or a mixture. Identify the surface first. Each needs its own preparation before emulsion.

Before you sand or scrape anything

Older paint can contain lead, and textured coatings applied before 2000 can contain asbestos. Do not sand or scrape a coating you are not sure about; get advice first.

Worked example

The example room, 4.2 m × 3.6 m × 2.4 m with one door and one window, has 34.484 m² of wall and 15.12 m² of ceiling: about 49.6 m² to wash down, and a 15.12 m² floor to sheet. Its edges, where the walls meet the skirting, the ceiling and the frames, come to about 41.4 m.

Masking the lot: one 50 m roll, or two 25 m rolls.
Cutting in with a brush instead: most of that tape is not needed.
Caulk: bought by the tube against the same 41.4 m. The size of the gaps decides how many.

The order of work

Clear the room and sheet the floor.
Wash down and let it dry.
Fill, then sand the filler flush.
Caulk the gaps and let it set.
Mask, if you are masking.
Mist coat or primer where it is needed, then cut in and roll the top coats.

Do it with BuildBatch

Open the Room Refresh Batch with paint chosen and answer the question about the walls. The preparation lines change to match, each marked Need or Check first, with why it is there and what decides the quantity.

What this guide does not cover

Quantities of filler, caulk or primer, which the walls decide and the products’ own coverage figures turn into tubs and tins. Preparing woodwork. Plastering or skimming a wall that is beyond filling. Diagnosing damp. Testing a coating for lead or asbestos, which is a job for someone qualified.