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How to Prep Walls Before Painting: Filling, Sanding and Priming
- amigospainters
- 5 hours ago
- 6 min read
To prep walls before painting, fill holes and cracks with a wall filler, let it dry, then sand smooth with 120 to 180 grit paper. Wash the walls with sugar soap, rinse, and let them dry. Prime any bare, patched, or stained areas before your topcoat goes on.
You've picked the colour, bought the paint, and you're keen to get the roller out. Here's the part nobody enjoys but everyone regrets skipping: the wall prep.
If you want to know how to prep walls before painting so the finish actually lasts, it comes down to four things done in order. Fill, sand, clean, then prime. Get those right and even a cheap paint looks sharp. Get them wrong and the best paint on the market will still flash, peel, and show every patch.
This is the exact process we use on Auckland homes, plus the mistakes we see most often when someone's had a go themselves first.
Why does prepping walls before painting matter so much?
Paint is thin. It's designed to bond to a clean, sound, slightly textured surface, not to hide problems underneath it. Whatever is on the wall before you paint, dust, grease, a glossy sheen, a hairline crack, will still be there after, just with a coat of colour sitting on top.
That's why the prep decides how long the job lasts. On a properly prepped interior wall you can expect a good repaint to hold up for years. Skip the prep and you can be looking at peeling or flaking within a season, especially in damp rooms.
Paint manufacturers are blunt about this too. Resene's own guidance is that the coating system is only as good as the surface underneath it, which is why their recommended paint systems all start with surface preparation, not the topcoat.
What do you need to prep walls before painting?
You don't need a van full of gear. A basic prep kit covers almost every interior job:
A tub of interior wall filler (lightweight filler for small holes, a setting-type compound for bigger repairs).
A flexible putty knife or two, one narrow and one wide for feathering out the edges.
Sandpaper or sanding sponges in 120 and 180 grit, plus a sanding block or pole for larger walls.
Sugar soap and a couple of clean sponges or cloths for washing the walls down.
A primer or undercoat suited to the surface, and painter's tape and drop sheets to protect everything else.
If your walls are freshly plastered or you've patched a big area, the order of the whole job matters as much as the tools. We break that down in our guide to plastering and painting in the right order.
How do you prep walls before painting, step by step?
Here's the process from bare wall to ready-to-paint. Work top to bottom and don't rush the drying times, that's where most DIY jobs come unstuck.
Clear and protect. Move furniture out, drop-sheet the floor, and take off switch and socket covers. Tape the edges of trim and ceilings you don't want painted.
Inspect the wall. Run a light across the surface at a low angle so it rakes over the wall. Dings, nail holes, and cracks throw shadows and become easy to see. Mark them with a pencil so you don't miss any.
Fill the holes and cracks. Press filler firmly into each spot with your putty knife, slightly proud of the surface, then scrape off the excess. Deep holes are better done in two thin layers than one thick one.
Let the filler dry fully. This is the step people rush. Wet filler sands into a gummy mess and shrinks as it cures, so leave it until it's hard right through, usually a few hours, longer for deep repairs.
Sand it smooth. Sand the filled spots and give the whole wall a light key with 120 to 180 grit. You're not stripping paint, just knocking off the sheen and feathering the patches flush so they disappear under the topcoat.
Wash the walls. Wipe everything down with sugar soap to lift grease, dust, and sanding residue, then rinse with clean water. Kitchens and hallways hold more grime than you'd think, and paint won't grip a greasy wall.
Prime where needed, then let it dry. Spot-prime bare filler, patches, and any stains, or prime the whole wall if you're making a big colour change. Once it's dry, you're ready for your first proper coat.
New in this house or first repaint? Our beginner's step by step guide to painting a room walks through the actual painting once the prep is done.
Do you always need to prime the walls first?
No, not every time. If you're putting a similar colour back onto a sound, previously painted wall that you've cleaned and lightly sanded, you can usually go straight to your topcoats.
You do want a primer or undercoat when any of these are true:
The surface is bare, so new GIB plasterboard, fresh plaster, or filled patches that will suck up paint unevenly.
You're covering stains, water marks, or a strong colour with a lighter one.
You're switching paint types, like going over old oil-based enamel with a waterborne paint.
Bare plasterboard is the big one in a lot of Auckland renos. Fresh GIB and plaster repairs are very porous, so they drink paint and dry patchy unless you seal them first. A wallboard sealer or undercoat evens out that absorbency, and Resene has a whole range of primers, sealers and undercoats matched to each surface.
Our take: the prep step most people underestimate
If we had to name the one step Auckland homeowners skip or rush, it's the washing and the drying either side of it. Filling and sanding feel like real work, so they get done. Wiping the walls with sugar soap feels optional, so it gets skipped.
It isn't optional. Kitchens carry a film of cooking grease, hallways and living rooms carry hand oils and dust, and bathrooms carry soap residue and often mould. Paint sitting on any of that can't bond properly, so it peels, or it flashes into shiny and dull patches once it dries. And painting over mould without treating it just lets it grow straight back through the new coat, which is a common story on shaded south-facing walls in our climate. If you're already seeing paint lift outside, that's a prep-and-repair job before anything else, and we cover it in our post on fixing peeling paint on weatherboards.
The other quiet killer is impatience. Filler that hasn't cured and primer that hasn't dried both trap moisture and wreck adhesion. Prep is mostly waiting, and the wall doesn't care about your weekend timeline. That's the honest part nobody puts on the tin.
Prepping walls before painting: frequently asked questions
Do I need to sand walls before repainting?
Usually yes, but only a light sand. On a previously painted wall you're just dulling the sheen and smoothing any filled spots so the new paint grips and the patches vanish. Use 120 to 180 grit and avoid anything coarser than 100, which can scratch the surface. Bare or very glossy walls need it most.
What grit sandpaper should I use on walls?
For general prep on interior walls, 120 to 180 grit is the sweet spot. Go slightly finer, around 180 to 220, for the final smoothing over filler or a sealer coat. Coarse papers under 100 grit leave scratch marks that show through the paint.
Should I use sugar soap or just water to clean the walls?
Sugar soap. Plain water pushes grease around rather than lifting it. Wash with sugar soap, then rinse with clean water so no residue is left, and let the wall dry fully before you prime or paint. Kitchens and bathrooms especially need it.
How long should filler dry before sanding and painting?
Give lightweight filler a few hours, and longer for deep or thick repairs, until it's hard right through. Sanding it too early clogs your paper and leaves a soft patch that shrinks later. When in doubt, wait, it's the cheapest insurance on the whole job.
Can I paint straight over old paint without prep?
Only if the existing paint is clean, sound, matte, and you're using a similar colour and paint type. Even then a quick wash and light sand gives a noticeably better result. Any flaking, gloss, grease, stains, or a big colour change means you need to prep and prime first. Our guide to painting a room covers the whole sequence.
Do I have to prime new plasterboard or plaster?
Yes. New GIB and fresh plaster are porous and will absorb paint unevenly, leaving a blotchy finish, so they need a wallboard sealer or undercoat first. This is one of the most common reasons a DIY repaint over a repair looks patchy, the bare patch was never sealed.
Want the prep done properly the first time?
Prepping walls is straightforward, but it's slow, fiddly work, and it's the part that decides whether your paint job lasts years or months. If you'd rather skip the sanding dust and the drying times, we handle the full job across Auckland, prep, primer, and finish coats, with the surface work done right.
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