A ridge in filler can look worse once it is painted, especially with daylight coming across the wall. Dust can stop paint gripping properly, and old wallpaper paste can reactivate under water-based paint. Start by finding out what the wall needs. A sound bedroom wall and a kitchen wall covered in grease need different preparation.
Start by working out what is on the wall
Look across the surface in daylight and with a lamp shining along the wall. Check for loose paint, cracks, grease, powdery filler, water staining and wallpaper paste. Wipe a small area clean and check it again when dry. If the surface still powders or rubs away, find out whether the problem is dust, a weak coating or the plaster itself before choosing a sealer. Paint will not hold loose material together indefinitely.
| What you find | What it usually needs |
|---|---|
| Sound, matt emulsion | Cleaning, local filling, light sanding and dust removal |
| Shiny or washable paint | Cleaning, preparing any gloss as specified and checking that the new paint will grip |
| Loose or peeling paint | Removing every unsound edge and finding the cause |
| Bare filler or plaster | Sealing absorbent areas as the paint instructions require |
| Paste residue | Thorough washing, rinsing and possibly a suitable sealer |
Tools and materials
- Dust sheets and masking materials
- Sugar soap or a suitable degreaser
- Filling knives and an appropriate interior filler
- Fine abrasive and a sanding block or pole sander; 120–180 grit is often useful for local filler repairs, depending on the filler
- Vacuum and a barely damp microfibre cloth
- Primer or sealer only where the surface calls for one
- Good task lighting
Prepare the wall in this order
- 1Clear and protect.
Move what you can, cover the floors and protect furniture and electronics from dust. Leave socket and switch faceplates in place unless someone competent can safely isolate and verify the circuit before removing them. Mask around them without getting cleaner or paint into the fittings.
- 2Wash contamination away.
Kitchens, hallways and areas around switches collect invisible grease. Work from the bottom up to control runs, rinse if the cleaner requires it and let the wall dry.
- 3Remove loose material.
Use a scraper to lift paint that has already failed. Do not keep digging into a sound edge. Feather the boundary with abrasive until it cannot be felt with a fingertip.
- 4Open and fill defects.
Check the crack before filling it. Minor, stable surface cracks can be prepared and filled as the filler instructions require. A crack that keeps reopening, widens or comes with loose plaster needs its cause assessed. Remove dust and fill within the product’s permitted depth rather than leaving a hump around the repair.
- 5Sand with light across the wall.
Sand dried filler and obvious nibs while moving the lamp. The target is a level transition, not a polished surface.
- 6Remove dust properly.
Vacuum first, including the top of skirting boards. Finish with a barely damp cloth where suitable. Fine dust is a common cause of poor adhesion and rough texture.
- 7Prime selectively.
Bare plaster, porous filler, stains and difficult glossy surfaces need different treatments. Use the product instructions for the actual substrate.
Close your eyes and run clean fingertips across repaired areas. Your hand often finds edges and ridges that are easy to miss while looking straight at a pale wall.
Common preparation mistakes
Skipping the wash because a wall “looks clean” is risky in kitchens and busy rooms. Filling over movement without investigating it can make the crack return. Untreated absorbent filler can leave dull patches because it takes up paint differently from the surrounding wall. The other extreme—priming the whole room with an unsuitable product—adds cost and can create compatibility problems.
Use the primer or thinned first coat specified for the paint and surface; do not substitute general-purpose PVA. New plaster, chalky paint and stains need different treatment. If you are stripping paper first, the wallpaper-removal guide explains why the paste matters as much as the paper. Widespread staining, loose plaster or torn plasterboard facing may need more than routine filling and painting.
Safety before sanding
Older paint can contain lead, and some older textured finishes contain asbestos. If either is suspected, stop before scraping or sanding and get competent advice about assessment and safe preparation. Do not treat hazardous coatings as an ordinary sanding job. For routine preparation, control dust and use suitable protection and stable access equipment rather than balancing on furniture.
Questions people ask
Do I need to sand every wall before painting?
No. Sound matt emulsion often needs cleaning and local preparation rather than aggressive sanding. Sand repairs, nibs and glossy areas, then remove the dust.
Can I paint the same day as filling?
Only when the filler is fully dry at its stated depth and conditions. Deep repairs can look dry at the surface while remaining soft underneath.
Should I wash walls with sugar soap?
It is useful where grease or grime is present. Follow the product instructions and rinse when required. Clean water and a suitable mild detergent may be enough for lightly soiled walls.