To prepare walls for painting, clean them, fill cracks and holes, sand the repairs flush, remove the dust and spot-prime any bare filler before the first coat. Done in that order, it gives paint a sound, even surface to grip.
Preparation is what decides whether a finish looks professional. Two decorators can use the same paint on the same wall, and the one who spent longer on prep will have the better result. This guide from PDS Painting Birmingham sets out the method we follow on every room, and the signs that a wall needs more than filler. It is part of our painting and decorating guides.
What You Need Before You Start
| Protection and cleaning | Filling, sanding and priming |
|---|---|
| Dust sheets for floors and furniture | Scraper and filling knife (narrow and wide) |
| Masking tape | Ready-mixed filler for small repairs |
| Sugar soap and a sponge | Powder filler for deeper holes |
| Buckets of clean water | Flexible filler or decorator’s caulk |
| Dust mask and eye protection | Sanding block with medium and fine abrasive paper |
| A torch for raking light | Primer, or emulsion to thin for spot-priming |
How to Prepare Walls for Painting, Step by Step
Work through these steps in order. Skipping one usually shows in the finish.
Step 1: Clear, Protect and Inspect the Room
Move furniture to the middle of the room and cover it, then sheet the floor right up to the skirting. Loosen or remove switch plates and sockets only if you isolate the electrics first; if you are unsure, mask around them or leave it to an electrician.
Then inspect. Hold a torch flat against the wall so the light rakes across the surface - every dent, crack and old repair will throw a shadow. Make a note of anything that is not a filling job: damp patches, mould, bubbling paint, plaster that sounds hollow when tapped, or cracks that are wide or stepped.
Step 2: Wash the Walls
Wash the walls with a sugar soap solution, working from the bottom up to avoid streaks, then rinse with clean water and let them dry. Kitchens and hallways need extra attention because grease and hand marks build up there.
Mould is different. It needs a fungicidal wash and the cause dealt with, not just a coat of paint over the top. Our guide to anti-mould paint and painting bathrooms explains how to treat it.
Step 3: Fill Cracks, Holes and Dents
The most common mistake is using one filler for everything. Match the filler to the crack:
- Hairline cracks and small dents - a fine surface filler, pressed in and smoothed off.
- Cracks that open and close - along the wall-ceiling joint, around door frames and where walls meet - a flexible filler or decorator’s caulk, not a rigid filler that will crack again.
- Deeper holes and old plug holes - rake out loose material, dampen or prime the hole if the filler’s instructions say so, and fill in layers rather than in one go.
- Larger dents and gouges - fill slightly proud of the surface so you can sand it back flush.
A useful trade tip: before filling a crack, rake it out into a shallow V with the corner of your scraper. That removes loose edges and gives the filler something to grip. Ceilings get the same treatment, and a cracked ceiling usually follows the joints in the plasterboard or old lath.
Follow the layer depth and drying time on the tub. Powder fillers mixed too wet, or built up too thick in one pass, tend to shrink and crack.
Step 4: Sand Repairs Flush
Once the filler is fully dry, sand it flush with a sanding block, starting with a medium grit and finishing with fine. Feather the edges so the repair blends into the wall, then check again with raking light and refill any low spots. Lightly key any glossy paint so the new coat grips.
Wear a dust mask. If your house was built before the 1970s, old paint may contain lead: do not dry-sand old paintwork until you have tested it, and follow HSE guidance on lead paint if the test is positive.
Step 5: Remove the Dust and Caulk the Gaps
Vacuum the skirtings and floor edges, then wipe the walls with a damp cloth so dust doesn’t end up rolled into the paint. Dust left on a wall makes the finish gritty and weakens adhesion.
Now run decorator’s caulk into the gaps between skirtings, architraves and the wall, and smooth it with a wet finger. It is a small job that makes the edges look crisp when painting skirting boards and woodwork - see our guide to painting skirting boards and woodwork. Let the caulk cure as the tube directs before painting over it.
Step 6: Spot-Prime Repairs and Stains
Bare filler and new plaster absorb paint differently from the painted wall around them. If you go straight over them, they “flash” through as dull patches. Spot-prime every repair with a primer or thinned emulsion first.
New plaster needs a mist coat on new plaster rather than full-strength paint.
Water marks, smoke and nicotine are a separate problem: they bleed back through emulsion however many coats you apply. They need a stain-blocking primer before the topcoats, and our guide on stain block for water and smoke stains explains how to use one.
Common Wall Preparation Mistakes
- Painting over dust or grease - the paint sits on the dirt, not the wall, and can peel or show marks.
- Using rigid filler in moving cracks - it looks fine for a few weeks, then the crack reopens.
- Not sanding filler flush - bumps and ridges show clearly once light hits the wall from the side.
- Skipping primer on filler - repairs flash through the topcoat as dull, patchy spots.
- Painting over damp or mould - the problem comes back through the new paint, usually quickly.
- Rushing filler that isn’t fully dry - it shrinks and sinks after painting, and the same goes for coats laid on too soon; read how long paint takes to dry and how many coats you need before you start.
When a Wall Needs More Than Filling
Filling and sanding is decorating preparation. Some walls need a different trade first, and it is better to know before you buy paint.
- Blown or hollow plaster, large holes, or walls that need a skim. If plaster sounds hollow, crumbles or has come away from the wall, filler won’t fix it. That is a plasterer’s job, and you’ll need a plasterer before the wall is decorated.
- Cracks that are wide, stepped, diagonal from window or door corners, or keep growing. These can point to movement in the building. Get them checked by a surveyor or structural engineer before anyone decorates.
- Active damp. Find and fix the cause - a leak, failed pointing, condensation - and let the wall dry out first.
For walls that need careful filling, sanding, stain-blocking and painting, a decorator can save you days of work and get a better finish. That is a big part of what we do with interior painting and decorating in Birmingham.
Preparing Outside Walls and Woodwork
Exterior masonry and render need different preparation: brushing down, a fungicidal wash, exterior-grade filler and sometimes a stabilising solution on chalky surfaces. Our guide to preparing masonry and render for painting covers it.
Related Painting Guides
- Mist coats and painting new plaster - how to seal fresh plaster before topcoats.
- Stain block paint - covering water, smoke and nicotine marks for good.
- Paint drying times and coats - when to recoat and how many coats you need.
- How long it takes to paint a room - realistic timings, prep included.
See all painting and decorating guides, or call 0121 752 9229 to get a quote for interior decorating.
Wall preparation questions
Do I need to sand walls before painting?
Sand every repair flush, and lightly sand any rough patches, nibs or glossy areas so the new paint grips. A previously painted wall in good condition usually just needs washing. Be careful with old paintwork in homes built before the 1970s: it may contain lead, so test it before sanding and follow HSE guidance.
What is the best filler for cracks in walls?
Match the filler to the crack. A fine surface filler suits hairline cracks and small dents. Cracks that open and close, such as along the wall-ceiling joint, need a flexible filler or decorator's caulk. Deep holes are best filled in layers with a powder filler, following the depths and drying times on the packaging.
Why do cracks keep coming back after I fill them?
Usually because the crack moves - with the seasons, with heating, or where two different materials meet - and a rigid filler can't flex with it. Rake it out and use a flexible filler or caulk. If a crack keeps widening, runs in steps or spreads diagonally from a window or door corner, have it checked before you redecorate.
Should I wash walls before painting?
Yes. Grease, finger marks and household grime stop paint sticking properly, especially in kitchens, hallways and around light switches. Wash with sugar soap, rinse with clean water to remove any residue, and let the wall dry completely before filling or painting.
Do I need to prime filler before painting?
Yes. Bare filler soaks up paint differently from the wall around it, so it shows through as dull patches, known as flashing. Spot-prime repairs with a primer or a thinned coat of emulsion first. Our guide on how long paint takes to dry and how many coats you need explains the coats that follow.
Can you paint straight onto new plaster?
Not with full-strength emulsion. New plaster needs to dry out fully first, then take a thinned mist coat that soaks in and gives the following coats something to bond to. Paint applied too thickly too soon tends to peel. See our guide to the mist coat on new plaster for the method.