How do I prepare walls for painting?

Paint magnifies whatever is underneath it. A wall that looks fine in normal light will show every unfilled dent and every unsanded ridge once it has a fresh coat catching the light. Preparation is unglamorous and it is the whole job.

What you'll need

• Sugar soap
• Filler and a filling knife
• Sandpaper (120 and 180 grit) and a sanding block
• Dust sheets and low-tack masking tape
• Primer or mist coat
• Work light (optional)

Step by step

1. Clear the room and cover what remains.Working around furniture doubles the time and guarantees a splash somewhere expensive.
2. Shine a work light across the wall at a shallow angle.Raking light reveals every dent and bump. Mark them with a pencil — you will not see them once you start.
3. Wash the walls with sugar soap, especially above radiators and in kitchens.Paint does not stick to grease, nicotine film or dust. This is the most-skipped and most-regretted step.
4. Scrape off any flaking paint and feather the edges.A hard edge between old paint and bare wall shows through the topcoat regardless of how many coats you apply.
5. Fill holes and cracks, leaving the filler slightly proud.All fillers shrink. Level now means slightly hollow tomorrow.
6. Sand filled areas flush with a flat block once fully dry.Flat block, light pressure. A fingertip digs a dish around the repair.
7. Vacuum and wipe the walls to remove sanding dust.Dust in the paint film is the most common cause of a gritty finish.
8. Prime bare plaster with a mist coat — emulsion thinned about 10% with water.New or newly exposed plaster is very absorbent. Undiluted paint sits on the surface and can peel.
9. Spot-prime all filled areas.Filler absorbs differently and will flash as a dull patch through the topcoat if left unprimed.

Tips

• Run your hand over the wall with your eyes shut before painting. You will feel defects you cannot see, and so will the paint.
• Mask sockets and switch plates rather than trying to cut around them, or unscrew the faceplates slightly and paint behind.
• If the wall has water stains, use a stain-blocking primer or they will bleed through however many topcoats you apply.

Common mistakes

• Skipping the wash — Grease and dust stop paint bonding — it peels or flashes later.
• Not priming bare plaster — The plaster drinks the paint, leaving a patchy finish that can peel.
• Sanding without checking with raking light first — You sand what you can see in flat light and miss what the fresh paint will reveal.

Questions people ask