Quick Answer
Proper wall preparation before painting involves five steps: fill any holes and cracks, sand smooth, clean the surface, apply primer where needed, then paint. Skipping prep is the single biggest reason paint jobs fail — peeling, flaking and uneven coverage are almost always caused by poor surface preparation, not poor paint.
Professional decorators will tell you the same thing: the finish is 80% preparation and 20% paint. In 2026, with premium paints costing £50–£100 per tin, it makes no sense to apply them to poorly prepared walls. Whether you're hiring a decorator or doing it yourself, this guide takes you through every step of proper wall preparation — the products to use, the common mistakes to avoid, and when you need professional help.
In this guide
Step-by-Step Wall Preparation Guide
Clear and protect the room
Remove furniture where possible or push it to the centre. Cover the floor with dust sheets — proper canvas dust sheets are worth using if you're doing multiple rooms, as paint spills happen. Remove switch plates, socket covers and curtain rails if possible.
Remove old wallpaper (if applicable)
If walls are papered, score the surface with a scoring tool and soak with a wallpaper stripper solution or warm water. Allow to soak for 10 minutes, then strip with a wide scraper. Stubborn paper may need a steam stripper. Once stripped, wash down walls with sugar soap solution to remove all adhesive residue.
Fill cracks and holes
Use a flexible filler (such as Polyfilla or Toupret) for small hairline cracks. For larger cracks (over 3mm), rake out loose material first and apply filler in two coats, allowing the first to dry before applying the second. For larger holes (e.g. from rawl plugs or removed fixtures), use a deep-fill compound. Allow all filler to dry completely before moving on.
Sand the surface
Once filler is completely dry, sand it flush with the surrounding wall using 80–120 grit sandpaper (or a sanding block). Also lightly sand any areas where paint is flaking or uneven. For woodwork, sand along the grain — use 120 grit to key the surface, then 180–240 grit for a smooth finish. Always wear a dust mask when sanding.
Clean the walls
Wash walls with a sugar soap solution (available from any DIY store) using a sponge. This removes grease, grime, nicotine staining and any remaining dust. Rinse with clean water and allow to dry completely before painting — ideally leave overnight. Don't skip this step even if walls look clean.
Prime where needed
Priming is needed in several situations: new plaster (use a diluted mist coat — see below), repaired areas (spot-prime filled areas), stain-blocking (over nicotine, water stains or marker pen — use a shellac-based stain blocker), bare wood (prime before gloss or eggshell), and highly absorbent surfaces.
Apply masking tape
Use low-tack decorator's masking tape around edges, skirtings, light switches and anywhere you want a clean line. Apply tape carefully and remove it while the final coat is still slightly tacky — this gives the cleanest edge.
Products You'll Need
| Product | Use | Approx. Cost |
|---|---|---|
| Sugar soap (liquid or powder) | Cleaning walls | £3–£7 |
| Flexible filler (Polyfilla/Toupret) | Small holes and cracks | £5–£15 |
| Deep fill compound | Larger holes | £8–£20 |
| Fine-grain sandpaper (80, 120, 240 grit) | Sanding filler and surfaces | £5–£15 for a pack |
| Sanding block or pole sander | Efficient sanding | £5–£20 |
| Dust sheets (canvas) | Floor and furniture protection | £10–£30 each |
| Low-tack masking tape (38mm) | Clean edges | £4–£8 per roll |
| Stain-blocking primer (shellac) | Over water stains, nicotine | £10–£20 |
| PVA mist coat (diluted) | New or bare plaster | £5–£12 |
| General-purpose primer/undercoat | Bare wood, fresh filler | £8–£18 |
Common Mistakes to Avoid
- Painting over damp walls: Paint will blister and peel. Always identify and fix the cause of damp first. See our guide on how to fix damp before repainting.
- Not allowing filler to dry completely: Wet filler under paint shrinks and cracks once it dries — and it can show through even two or three coats of paint.
- Skipping the sugar soap wash: Grease and grime prevent paint from bonding. Even walls that look clean will have a layer of household grease near kitchen doorways and finger marks near switches.
- Painting new plaster without a mist coat: Fresh plaster is highly absorbent and will suck the moisture out of standard emulsion, causing it to dry powdery and patchy. Always mist coat new plaster first — dilute standard emulsion 70/30 with water.
- Applying gloss directly to bare wood: Bare wood must be primed first. Knots must be treated with knotting solution before priming — otherwise resin bleeds through and causes discolouration.
- Removing masking tape after paint has dried: If paint has dried fully, the tape can pull it off in strips. Remove tape while the final coat is still slightly tacky (usually 30–60 minutes after applying), pulling it at 45° away from the painted edge.
Special Cases
New or Freshly Plastered Walls
New plaster must be left to dry completely — typically 4–6 weeks for a fresh skim coat, longer for full-depth plaster. It should change from a dark brown to a uniform light colour before painting. Once dry, apply a mist coat (diluted emulsion: 70% emulsion, 30% water) before painting with full-strength emulsion. If walls need plastering before you can decorate, see our guide on plastering costs in the UK.
Textured or Artex Walls and Ceilings
Old Artex or textured coatings can be painted over — roll on a thick emulsion and the texture will be painted in place. If you want a smooth finish, you have options: skim plaster over the top (best result, adds cost), apply a textured filler to fill in the pattern, or use a specialist textured wall lining paper. Very old Artex (pre-1985) may contain asbestos — do not sand or mechanically remove it without testing first.
Damp Walls
Painting over damp walls is one of the most common and costly mistakes in decorating. The paint will fail within months. Always identify the type of damp (rising, penetrating or condensation), treat the cause, and allow walls to dry completely before redecorating. Our full guide covers how to fix damp before repainting a room.
When to Call a Professional
Wall preparation is well within DIY capability for most standard rooms. However, consider hiring a professional decorator (or plasterer first) if:
- Walls have significant cracking or movement
- There are signs of persistent damp
- Walls need re-plastering or re-skimming (needs a plasterer, not a decorator)
- You're dealing with high ceilings or stairwells
- There are suspected asbestos-containing materials
For cost information on professional interior painting, see our guide to interior painting costs in the UK. For help choosing between DIY and professional decorating, read our guide: should you paint your own house or hire a decorator?
Frequently Asked Questions
How long should you wait before painting over filler?+
Most ready-mixed fillers are dry enough to sand and paint over within 1–4 hours for small areas, but deeper fills may need 24 hours. The filler should feel completely hard and change colour uniformly before you sand it. Painting over wet filler is one of the most common causes of poor results — when in doubt, leave it overnight.
Do I need to sand walls before repainting?+
Not necessarily for a full repaint over existing sound paintwork. However, you should lightly sand any raised edges where old paint has chipped, and any filled areas. If existing paint has a sheen (silk or eggshell), lightly keying the surface with fine sandpaper improves adhesion of new paint.
What is sugar soap and is it necessary?+
Sugar soap is an alkaline cleaning solution that cuts through grease and grime on painted surfaces. It's necessary for any surface that has been lived in — cooking, touching walls, cigarette smoke and general household grime create an invisible layer that prevents paint from bonding well. It's inexpensive and the step takes 30–60 minutes per room — it's always worth doing.
How do I prepare walls with lots of picture hooks and nails?+
Remove all fixtures. Fill each hole with a small amount of filler — a pointing trowel or even your finger works well for small holes. Allow to dry, sand flush, and apply a spot prime before painting. Use a small brush or roller sponge to apply the spot prime so it doesn't create a visible patch.
How do I deal with woodchip wallpaper?+
Woodchip paper can be painted over if it's firmly adhered. Score lightly and soak to test — if large sections lift, stripping is the better option. If you want to remove it, score it thoroughly with a scoring tool, soak with warm water or a wallpaper stripper solution, and strip with a wide scraper. Woodchip often has multiple layers and can be very time-consuming to remove.
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