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HOW-TO GUIDEHow to Clean Walls Before Painting So the Paint Actually Sticks
Most paint jobs that peel, streak or dry patchy were doomed before the tin was opened. A weak sugar soap solution, a clean water rinse and a proper drying window will do more for your finish than upgrading to a premium emulsion. The trick is getting walls clean without soaking the plaster or leaving a soapy film behind.
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Emulsion needs a clean, slightly keyed surface to grip. What sits on a typical wall is invisible until you paint over it: cooking grease that drifts well beyond the kitchen, skin oils around switches and door frames, smoke residue and dust at picture rail height.
Water based paint reacts badly to all of it. On greasy patches it beads slightly as you roll, leaving a duller sheen once the wall dries. On dusty walls the paint bonds to the dust rather than the substrate, which is why a fingernail can lift it months later.
What you need
- Sugar soap, either concentrate to dilute or a ready-mixed spray
- Two buckets, one for the solution and one for clean rinse water
- A sponge or microfibre cloths, plus a lint-free cloth for drying
- Dust sheets and masking tape for sockets and switches
- Rubber gloves and eye protection, since sugar soap is alkaline
- A soft brush or vacuum brush head for the initial dust down
The order that works
- Clear and protect first. Move furniture into the middle of the room, cover the flooring, and mask off sockets and switches. Turn the power off at the consumer unit if you plan to wipe close to fittings.
- Dust before you get anything wet. Run a soft brush or vacuum brush head over the wall top to bottom, including cobwebs in corners and the ledge on top of skirting and architrave. Wetting dust turns it into a smear you then have to chase around.
- Mix the solution weak. Follow the dilution on the bottle and lean towards the weaker end. A strong mix leaves an alkaline residue that interferes with adhesion, which is the opposite of what you want.
- Wash from the bottom up. Wring the sponge out so it is damp rather than dripping and work in overlapping sections of roughly a square metre.
- Rinse as you go. Follow each washed section with a second pass of clean water and a wrung-out cloth, changing the water as soon as it looks cloudy. This is the step people skip, and it decides whether you end up with a film or a clean surface.
- Spot-treat the stubborn areas. Grease near the cooker, mould in bathroom corners and scuffs behind doors need a second targeted pass rather than more scrubbing everywhere else.
- Let it dry fully. Ventilate the room and give it real time before you open a tin.
Quick Tip
Always wash upwards, not downwards. Dirty water running down a dry wall soaks in and leaves streaks that are hard to shift and often ghost through your first coat. If the wall below is already damp and clean, the runs have nothing to stain.
The awkward spots
Kitchen grease
Walls near a hob build up a film that ordinary washing smears rather than removes. Use a slightly stronger mix on those areas only, give it a minute to work instead of scrubbing hard, then rinse twice. If paint still beads when you cut in, the grease is not gone.
Bathroom mould
Treat it with a proprietary mould remover before anything else and let it dwell for the time on the label. Painting over live mould traps it and it comes back through. Mould is a ventilation symptom, so cleaning it is a treatment rather than a cure.
Nicotine, soot and water stains
These are soluble and will bleed through. Clean them back as far as you reasonably can, then seal with a stain-blocking primer. One coat of a proper blocker beats three coats of white emulsion every time.
Scuffs and crayon
A melamine eraser lifts most marks but burnishes the surface. Scuff that patch lightly with fine abrasive so the paint has something to grip.
How long should walls dry before painting?
A light wipe down in a warm, ventilated room is usually touch dry in two to four hours. A full wash deserves overnight. Cold rooms, north-facing walls and lime plaster all take longer, and a closed-up room with no airflow can stay damp for a day or more.
If you are unsure, hold a clean palm flat against the wall for a few seconds. Cool and slightly clammy means it is still holding water.
When a full wash is overkill
Freshly plastered walls do not need washing, they need dust removal and a mist coat. Bedroom walls painted a couple of years ago and untouched since are usually just dusty, so a vacuum brush and a damp cloth around switch plates is plenty. Save the buckets for kitchens, bathrooms, hallways and stairwells.
Mistakes worth avoiding
- Mixing sugar soap strong and skipping the rinse, leaving the residue that causes the very adhesion problem you set out to prevent
- Soaking wallpaper seams or lining paper, which lifts edges you then have to re-glue
- Painting a wall that feels dry on the surface but is still wet behind
Do I really need sugar soap, or will washing up liquid do?
Washing up liquid handles light dust, but many formulations contain moisturisers that leave a residue. If you use it, keep the mix very weak and rinse twice. Sugar soap breaks down grease and rinses away cleanly, which is why decorators default to it.
Can I paint over a wall that has just been washed?
No. A damp surface affects how emulsion flows and cures, giving patchy sheen and slow drying. Allow a few hours after a light wipe down and overnight after a proper wash, with a window open and some warmth in the room.
Should I wash walls before or after filling and sanding?
Wash first, then fill, then sand, then dust off. Filler will not bond to a greasy wall, and sanding a wall you have just washed creates slurry rather than dust. A vacuum and tack cloth after sanding is all you need before painting.