Preparing Walls Before Painting a Room

Why Preparation Makes or Breaks the Finish

Paint is a surprisingly thin film. Brushed or rolled onto a wall, a coat of emulsion is often no thicker than a sheet of paper, which means it hides very little. Every ridge, dent and hairline crack will still be visible once the paint dries, and often more so, because a fresh flat colour catches light and throws shadows into imperfections you barely noticed before.

Good preparation also affects how long the job lasts. Paint grips best to a sound, clean, slightly textured surface. Applied over flaking old paint, greasy kitchen walls or dusty filler, it can blister, peel or flake within months. Spending a weekend on prep is the difference between a finish that looks professional for a decade and one you will be touching up next spring.

Empty the Room and Give the Walls a Proper Clean

Before you touch a filling knife, clear the space as much as you can. Move furniture out or into the centre of the room, lay dust sheets over the floor and anything you cannot shift, and take down pictures, mirrors and curtain poles. Switch off the relevant circuit at the consumer unit, then remove socket and light switch faceplates, popping the screws into a mug so they do not vanish.

Washing the walls is the step most people skip, and it is the one that causes the most callbacks. Sugar soap, mixed to the strength on the packet, cuts through cooking grease, nicotine film, fingerprints and general grime. Work from the bottom up so drips do not streak down clean sections, paying extra attention around light switches, kitchen splashbacks and anywhere a hand regularly lands. Rinse with clean warm water and let everything dry completely, which may take a few hours in a cool room.

While the walls dry, run your hand across them. Any paint that sounds hollow or lifts under a scraper needs removing back to a firm edge, then feathering with sandpaper so the repair blends in.

Fill Cracks, Holes and Dents Properly

Choose your filler to suit the job rather than grabbing whatever is in the shed.

• Powder filler for deep holes, chased-in cables and larger repairs. Mixed fresh, it sets hard and sands well.
• Lightweight ready-mixed filler for shallow dents and hairline cracks. Convenient and easy to sand, but it shrinks, so give it a second pass if needed.
• Decorator's caulk for movement gaps where walls meet ceilings, skirting boards, architraves and window reveals. It stays flexible, so it will not crack as the building moves.

Apply filler with a knife wider than the repair so you can press it in and strike it flush in one movement. Slightly overfill and let it set proud of the wall, then sand back level; trying to fill perfectly flat in one go rarely works. For caulk, run a neat bead, smooth it with a damp finger, and wipe the excess away before it skins over — caulk does not sand well. Remember that filler shrinks as it dries, so a second thin application after a couple of hours gives a far better result than one thick dollop.

Sand, Feather and Dust Off

Once the filler is fully dry, sand the repairs with 120-grit abrasive paper wrapped around a sanding block, keeping the block flat to avoid dishing the wall. Feather the edges of the repair out into the surrounding paint so there is no discernible lip. Finish the whole wall with a light pass of 180 to 240 grit to key any existing paint and dull any sheen.

Glossy surfaces need particular attention. Old silk or eggshell paint can be too smooth for new emulsion to grip, so dull it to a flat finish, either with fine abrasive or a liquid de-glosser, then wipe clean. After sanding, brush or vacuum the walls down and wipe them with a barely damp cloth. Dust left behind will show through the first coat as a gritty texture, and no amount of extra paint will hide it.

Prime Bare Patches and Tricky Areas

Primer is not optional on absorbent or stained surfaces. Bare filler and new plaster drink moisture out of emulsion at different rates to the painted areas around them, which is why patches can flash or look patchy forever. A mist coat — emulsion thinned roughly one part water to four parts paint — seals new plaster and gives subsequent coats an even base. Alternatively, use a dedicated plaster primer.

Stains need a different approach altogether. Water marks, smoke damage, crayon and ink will bleed through standard emulsion, so seal them with a stain-blocking primer and allow it to dry fully before painting. On very porous or previously distempered walls, a coat of primer over the whole surface will save you from applying four coats of topcoat.

Final Checks Before the First Coat

Run a hand across the wall one last time, ideally in raking light from a lamp held close to the surface. Touch in any spots you missed, then dust down again. Mask skirting boards, door frames and window reveals with low-tack tape, press it firmly, and lay out your paint, tray, roller and brush so you are not hunting for anything mid-wall. With the surface clean, sound, filled, sanded and primed, the paint itself becomes the easy part — and the finish will show it.