Preparing Interior Walls Before Applying a Fresh Coat

Preparation Is the Job That Makes the Job

A fresh coat of emulsion can transform a tired room in an afternoon, but the finish you get is almost always decided before the first drop of paint leaves the tin. Paint is unforgiving: it highlights every ridge of old filler, every smear of grease and every hairline crack in the plaster. Ten minutes spent filling and sanding properly will save you hours of touching up later, and it means the colour you chose will look as good in five years as it did on day one.

The good news is that preparation is not complicated. It is mostly cleaning, filling and smoothing, done in the right order and with a bit of patience between stages.

Clear the Room and Protect What Stays

Empty the space as far as you reasonably can. Large furniture out, pictures and curtain poles down, radiator valves wrapped in a bit of cling film and masking tape. Switch off the power at the consumer unit if you're removing socket or switch fascias, and tape the screws to the plate so they don't vanish into the carpet.

Cover floors with dust sheets rather than thin plastic, which becomes slippery underfoot once there's plaster dust about. Tape down the edges. If you're working near a fitted carpet, a sheet of polythene under the dust sheet gives extra protection against paint and filler spills.

Fill Cracks, Holes and Gaps

Work systematically around the room, from the ceiling down. Different defects need different fillers:

• Hairline plaster cracks: rake the crack out slightly with a filling knife or scraper so the filler has something to grip, then press in fine surface filler. Wide, shallow cracks reappear if you simply skim over the top.
• Screw and picture hook holes: a dab of lightweight ready-mixed filler is enough. Don't overfill; a fingertip of filler pushed in flush is easier to sand than a proud blob.
• Deep holes and chunks of missing plaster: build up in layers of no more than 5mm at a time, letting each layer dry. Powder filler mixed to a stiff paste is ideal here. Fill slightly proud of the surface and sand back flush.
• Gaps between skirting, door frames and walls: filler will crack. Use a decorator's caulk, run a thin bead, then smooth it with a damp finger or a caulking tool. Wipe the excess immediately, because caulk sets like rubber and doesn't sand nicely.

If you find damp staining, blown plaster or crumbling render, stop and deal with the cause before decorating. Painting over a damp problem only seals the moisture in.

Sand, Feather and Remove the Dust

Once filler is thoroughly dry, sand it flush with the surrounding wall. A medium grit around 120 is right for filler, while 180 to 240 works well for keying previously painted surfaces. Wrap the paper around a sanding block for flat areas, and use a pole sander for ceilings and upper walls to save your shoulders.

The trick with sanding is "feathering": blending the edge of the repair out into the surrounding wall so you cannot feel a step with your fingertips. Sand in small circles and check often by running a hand over the area. If you're working on old gloss or satin paint, a light key with fine paper gives the new paint something to bite into.

Wear a dust mask and eye protection, and open windows. When the sanding is done, vacuum the walls with a brush attachment if you have one, then wipe everything down with a damp cloth. Dust left on the wall is the single most common reason new paint feels gritty.

Wash Away Grime, Grease and Stains

Paint will not stick to dirt, and dirt hides in the places you least expect. Washing down is quick and makes an enormous difference.

• Mix a solution of sugar soap and warm water, and work from the top of the wall downwards with a sponge or cloth. Rinse with clean water and let the walls dry completely.
• In kitchens, pay particular attention to the area around the hob, where a fine film of cooking grease settles. A degreaser or a second wash with sugar soap will shift it.
• In homes where smoking has taken place, wash the walls twice, then apply a stain-blocking primer. Tobacco tar bleeds through ordinary emulsion and will reappear as a yellow bloom within weeks.
• Mould in bathrooms should be treated with a fungicidal wash, then covered with a stain-blocking primer before painting with a mould-resistant paint. Improve ventilation as well, or the problem returns.

Allow at least a few hours, ideally overnight, for walls to dry fully before you think about painting. Damp walls trap moisture under the film and cause flaking later.

Prime Patches and Check Before You Paint

Newly filled areas and bare plaster soak up paint at a different rate from the rest of the wall, which is why patchy, dull spots appear even after two coats. Spot-prime filler and caulk with a thinned coat of emulsion, sometimes called a mist coat. On fresh plaster, a full mist coat of emulsion thinned with around 10 to 20 per cent water is essential; it seals the surface and gives the finish coat an even base.

Finally, walk the room with the light behind you and look along the walls at a raking angle. Small blemishes that are invisible in normal light show up clearly at this angle. Run your fingertips over each repair. If anything feels rough, proud or hollow, deal with it now.

When the room is clean, dry, smooth and properly primed, you can pick up your brush with confidence. That's the point at which painting stops being a gamble and becomes the easy part.