How to prep walls for painting

What prep actually means: a sound surface before a single coat

Prep is the unglamorous half of a paint job, and it's the half that lasts. It comes down to four moves. Clean the surface so the paint sticks to wall and not to grime. Fill the cracks and holes so they don't telegraph through the finish. Sand back anything loose, flaking or shiny so the new coat has something to bite into. Then prime, so the topcoat goes on even. Do those and the colour has a sound surface to grip.

On a tired or older wall that work is a job in its own right: patch and paint. It's a real, priced line of work on SureQuote, with options that scale from a few simple repairs up to heavier work on older, worn surfaces. That's worth knowing before you compare quotes: a cheap number that skips the prep isn't pricing the same job as one that includes it. The details of what prep involves on a specific wall (how to fill, what primer, how far to sand back) are their own walk-through, so for the full method see the guide on patching and repairing walls before painting.

The reason it pays off comes straight from the people who make the paint. A premium exterior system like Dulux Weathershield is guaranteed against peeling, flaking and blistering only when it's 'applied and maintained in accordance with the product instructions, to a suitable and sound substrate'.1DuluxView source Read that the other way around: the durability rides on the prep, not the tin.

Three problems send people here, and the fix depends on which one you've got

Most people land on prep because something's already wrong with the surface. It almost always falls into one of three problems, and naming yours is the first step, because each one points to a different fix. Here's the map; each has its own full guide below.

Cracks, holes and tired walls are the straightforward one. A wall that's been knocked, patched, or just worn smooth needs filling, sanding and repainting: that's patch-and-paint work, and a painter handles it as part of the job. The walk-through for how to patch so the repair doesn't show is in the guide on patching and repairing walls before painting.

Paint that peels, flakes or blisters is the second, and it's a sign the surface let go rather than the colour. It usually traces back to moisture in the wall, painting over a damp or glossy surface, or the wrong primer underneath.2DuluxView source The detail on each cause, and what each one means for the fix, is in the guide on why paint peels, flakes or blisters.

Mould, damp and stains are the third, and they're a moisture problem before they're a paint problem. Mould grows where there's excess moisture and poor airflow, and a fresh coat over it just feeds the same patch again; significant mould across several spots is a job for a specialist, not a paint tin.3Dulux TradeView source The guide on mould, damp and stains on painted walls covers how to tell a surface stain from a moisture source and what to do about each.

Some problems are a painter's job; some need another trade first

Knowing the problem is half the work. The other half is knowing who fixes it, because not everything that shows up on a wall is a painting job. Patching, filling, sanding back peeling paint and recoating are squarely painter work, and they feed straight into the interior or exterior repaint once the surface is sound.

Damp is the line to watch. Mould or staining that keeps coming back, rising moisture, or large areas of mould usually point to a cause behind the wall: a leak, a drainage problem, poor ventilation. That source needs the right trade to fix it before any paint goes on, or the problem comes straight back through the new coat.3Dulux TradeView source A painter can prep and repaint a sound surface; a recurring damp source is a different job, priced on its own.

Older homes carry one more first step. Houses built before 1970 often used high-lead paint. Paint made before 1965 could hold as much as 50 per cent lead, and the limit only fell to 1% in 1965, 0.25% in 1992 and 0.1% in 1997.4DCCEEWView source Lead is a health hazard, and sanding or scraping is exactly what disturbs it. So on a pre-1970 home the safe move is to have the old paint checked by a licensed professional before anyone preps it. It is not a DIY task.

Why prep is where the money is, and what it saves you

When two quotes for the same wall come back miles apart, prep is usually the reason. The patching, scraping and cleaning that gets a surface sound is labour, and on an older or worse-condition wall it's a lot of it, so it moves the price more than any other part of the job, the paint included. A clean, sound wall is a simple recoat; a flaking, cracked one is a strip-back-and-repair job before the finish even starts.

Skimping there is a false economy. Paint over a loose or damp surface and the new coat lifts with the old, and you're back inside a year or two paying to redo it. The durability you bought rides on the prep, not the colour.1DuluxView source Your real number depends on your own walls and their state, so the most useful thing is a range built from your home, not a headline price. Use the estimate below, then see the full breakdown on the cost guide.

What will your prep and repaint cost?

Where to go next: the full guide for each problem

This page is the map. Once you know which problem you've got, its full guide takes you the rest of the way. For surface repairs, see patching and repairing walls before painting. For a coat that's lifting, see why paint peels, flakes or blisters. For damp, mould or water marks, see mould, damp and stains on painted walls.

And if you're working out the whole job, not just the prep, this sits inside the bigger picture. The complete guide to house painting covers interior, exterior, roof, cost and choosing paint and a painter.