Every decorator who's been doing this for more than a few years will tell you the same thing about prepping walls for painting: the paint itself is rarely what goes wrong. It's what's underneath it. Paint applied over grease, dust, an unfilled crack or a damp patch doesn't fail because the paint was poor quality. It fails because it never had a sound surface to grip onto in the first place.
That's the frustrating part about a botched paint job. The bubbling, the flaking, the patch that looks fine for three weeks and then lifts clean off the wall, none of it shows up while you're painting. It shows up afterwards, when it's far more work to put right than it would have been to prevent. If you're about to redecorate a room or repaint the outside of the house, the hour or two you spend on preparation is the difference between a finish that lasts and one you're doing again next year.
This guide covers the four stages that matter: cleaning, filling, treating damp, and priming, in the order you should actually tackle them.
Cleaning and Degreasing: The Step Everyone Skips
It's tempting to go straight from "the wall looks fine" to "cut in around the edges", but most walls carry more than you'd think. Kitchens and hallways in particular pick up a fine layer of airborne grease from cooking, along with dust, cobwebs and, on older paintwork, a slight chalky residue from the previous coat breaking down.
Paint is designed to bond to a clean, stable surface. Any layer of grease or dust sitting between the wall and the new coat becomes the weak point: the paint bonds to the dust rather than the wall, and it's only a matter of time before that thin, dusty layer lets go and takes the paint with it.
What to do:
- Dust down the whole wall first, working top to bottom, including coving, skirting boards and any decorative trim, so you're not redepositing dust onto a surface you've already cleaned.
- Wash kitchens, hallways and bathrooms with warm water and a mild degreasing detergent (sugar soap is the standard choice for a reason). Rinse with clean water afterwards so no residue is left behind, and let the wall dry fully before moving on.
- For exterior walls, a wash down removes algae, dirt and general grime that's built up since the last coat. If you've got noticeable green or black staining on brickwork or render, treat this properly rather than painting straight over it, since it will usually reappear through emulsion or masonry paint within a season.
- Lightly sand any glossy or previously painted surface once it's clean and dry. A key (a slightly roughened surface) gives the new coat something to grip. Our range of sand paper and abrasives covers everything from a fine finishing grade for smoothing filler to a coarser grade for keying gloss trim.
Filling and Smoothing: Getting a Flush Finish
Cracks, nail holes, dents from the last set of shelf brackets, and the hairline settlement cracks every house develops over time all need dealing with before paint goes anywhere near them. Paint doesn't hide imperfections. Under the wrong light, it highlights them.
Matching the filler to the job.
- Fine cracks and small nail holes: a lightweight, ready-mixed filler applied straight from the tub, built up in thin layers if the hole is deeper than a couple of millimetres.
- Larger cracks, dents or areas of damaged plaster: a powder filler mixed to a firmer consistency, applied with a wider filling knife and built up in stages so it doesn't slump or crack as it dries.
- Recurring hairline cracks, particularly around door frames and ceiling joins: a flexible filler is worth the extra cost, since it moves slightly with the building rather than cracking open again within a few months.
Our fillers range covers both types, from quick-drying interior fillers to more heavy-duty options for exterior render and masonry repairs.
Getting it flush. Overfill very slightly, since most fillers shrink a little as they dry, and a sunken patch will show through paint just as clearly as a raised one. Once fully dry, sand back with a fine grade until the repair is completely flush with the surrounding wall, checking with a hand run flat across the surface rather than relying on sight alone; a shadow you can feel but not quite see will still show once paint catches the light. Dust off thoroughly before moving on to the next stage.
Dealing with Damp and Stains: Don't Paint Over the Problem
This is the step that causes the most expensive mistakes, because painting over a damp patch or a stain doesn't fix it. It hides it for a matter of weeks before the moisture works its way back through the new coat, often bringing a yellow-brown tide mark with it.
Why ordinary paint won't cover it. Standard emulsion and masonry paint are not designed to block moisture or the salts and tannins that damp and staining leave behind. If there's an active source of damp (a leak, penetrating damp through render, or rising damp at low level), painting over it without addressing the cause is only ever a temporary fix, and the underlying issue needs sorting first.
What a damp seal does. Once you're confident the source of the damp itself has been dealt with, a dedicated damp seal or stain block creates a barrier coat that prevents residual moisture and staining from bleeding through into your new paintwork. This is a genuinely different product to a primer or undercoat: it's formulated specifically to block what's underneath rather than just to help the next coat bond. Our damp seal products are designed for exactly this: old water stains, nicotine staining, and general problem areas where ordinary paint has failed before.
When to use it:
- Any visible tide mark or brown staining, even if the wall feels dry to the touch.
- Ceilings or walls below a bathroom, where minor historic leaks are common even after the plumbing issue itself has been fixed.
- Chimney breasts and areas prone to condensation, where damp patches tend to reappear seasonally.
The Priming Rule: When It's Genuinely Non-Negotiable
Primer and mist coats get skipped more than almost any other step, usually because they feel like an extra coat for no visible difference. In a handful of situations, though, skipping it is what causes the finish to fail.
Mist coats on new plaster. Fresh plaster is highly absorbent. Applied straight, ordinary emulsion sinks into the surface unevenly, drying patchy and dull, and the plaster keeps drawing moisture out of every subsequent coat too. A mist coat, standard emulsion thinned with around 10 to 20% water, seals the plaster's absorbency so the full-strength topcoats that follow go on evenly and hold their finish. This is genuinely not optional on new plaster; it's the difference between a flat, even wall and a patchy one that never quite looks finished.
Priming over filler and bare masonry. Filled areas are more absorbent than the surrounding painted wall, and if you paint straight over them without priming, they'll often show through as a slightly different sheen or shade, sometimes called "flashing". A quick coat of primer over filled patches evens out absorbency before the topcoat goes on.
Priming over a stark colour change. Going from a dark colour to a much lighter one (or vice versa) generally needs a primer or undercoat to stop the old colour bleeding through, and it usually saves you a full extra topcoat compared with trying to cover it in emulsion alone.
Exterior masonry. Bare brick, render or previously unpainted masonry is far more porous than an interior wall, and a stabilising primer or the first coat of a proper masonry paint system controls how much the surface draws in the topcoat, giving a more even, longer-lasting finish. For larger exterior jobs or repeat coverage work, our contract paint range is worth a look for trade-grade coverage at a lower cost per litre.
Getting the Order Right
If you take one thing from this guide, it's that the order matters as much as the individual steps: clean and degrease first, fill and sand second, deal with damp and staining third, and prime last, immediately before the topcoat goes on. Skip a step, or do them out of order, and you're relying on the paint itself to compensate for a surface it was never designed to cover.
None of this adds more than an hour or two to most rooms, and considerably less than the time it takes to redo a job that's failed within the year. Once the wall is clean, sound, sealed and primed, the actual painting is the easy part.
Pop into your local DBM branch for advice on the right filler, damp seal or primer for your specific job, or browse our fillers, sand paper and abrasives, damp seal, contract and masonry paint, and decorator tools ranges online.