Paint magnifies defects. A ridge that seems minor on bare plaster can throw a shadow after two coats; a dusty patch can release under the roller; old paste can reactivate and make fresh emulsion craze or peel. The reliable approach is to diagnose the wall first, then do the preparation the condition actually requires.
Start by working out what is on the wall
Look across the surface in daylight and under a lamp held close to the wall. Check for loose paint, hairline cracks, shiny or greasy areas, powdery filler, water staining and wallpaper-paste residue. Rub a dark cloth over a pale wall: if it comes away chalky, the surface may need stabilising or sealing rather than another blind coat of emulsion.
| What you find | What it usually needs |
|---|---|
| Sound, matt emulsion | Cleaning, local filling, light sanding and dust removal |
| Shiny or washable paint | Degreasing, dulling the sheen and an adhesion check |
| Loose or peeling paint | Removing every unsound edge and finding the cause |
| Bare filler or plaster | Spot-priming so the repair does not flash through |
| 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
- 120–180 grit abrasive with a sanding block or pole sander
- Vacuum and a barely damp microfibre cloth
- Primer or sealer only where the surface calls for one
- Good task lighting
A dependable preparation sequence
- 1Clear and protect.
Move what you can, cover floors fully and isolate dust-sensitive items. Remove switch plates only if you can safely isolate the circuit; otherwise mask neatly around them.
- 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.
Only widen cracks slightly when they are genuinely stable. Remove dust, fill in thin layers and avoid building a large 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. Painting straight over bare filler creates dull patches because filler absorbs paint differently. The other extreme—priming the whole room with an unsuitable product—adds cost and can create compatibility problems.
Do not use PVA as a general paint primer. It can form a smooth film that emulsion does not reliably bond to. For new plaster, follow the paint manufacturer’s dilution instructions or use a specified plaster primer; for chalky or unstable surfaces, use a product designed for that exact condition. If the wall has old paste, widespread staining or damaged plasterboard, the preparation may be more than a simple DIY repaint.
Safety before sanding
Old coatings can contain lead. Minimise dust, use wet abrasion or on-tool extraction where appropriate, contain debris and seek competent advice if the coating may be hazardous. Do not sand textured coatings until asbestos risk has been assessed. For high areas, use 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.