Start with an Honest Look at the Wall

Before you open a tub of filler, spend ten minutes working out what you are actually dealing with. Run your hand across the surface, look at it in raking light from a window or a work lamp, and tap around any suspect patches. Hollow-sounding plaster, powdery areas, and dark stains all need attention first — paint over them and they will reappear within weeks. Fill-and-paint jobs only look professional when the wall is sound underneath.

Clear the room properly. Move furniture out or into the middle and cover it, lay dust sheets, and tape around sockets, switches and skirting. Small grit from sanding gets everywhere, and once it is in a carpet it stays there.

Gather your kit before you start so you are not stopping mid-job:

• A filling knife or broad scraper, plus a smaller one for tight corners
• Lightweight flexible filler for hairline cracks and shallow dents
• Two-part or powder filler for deep holes and chases
• Sanding blocks in 120, 180 and 240 grit, plus a sanding sponge for corners
• Fine-surface filler or a ready-mixed skim for wide, shallow hollows
• Dust mask, eye protection, a tack cloth and a hoover with a brush head
• A quality synthetic brush, a roller with a medium-pile sleeve, a roller tray and a pole
• Enough emulsion to do two full coats, plus a little extra for spot-priming

Filling Cracks and Holes the Right Way

Flexible filler is your best friend on interior walls. Modern lightweight fillers stay slightly pliable when dry, so they move with the wall instead of cracking open again the first time the heating comes on. That flexibility is why they work so well over plasterboard joints and around door frames, where a rigid filler would simply snap.

For hairline cracks, open them up slightly with the corner of your filling knife or a scraper. This sounds counterintuitive, but filler needs somewhere to sit — a crack that is closed tight will only take a smear on the surface, which will flake off. Once opened, press the filler in firmly across the crack, then scrape the knife flat against the wall to remove the excess. Aim to leave the filler very slightly proud rather than digging it out level.

Deep holes need a different approach. If you fill a 20mm hole in one go, the filler shrinks as it dries and you get a sunken crater. Build it up in two or three thin layers instead, letting each one set. For anything deeper than about 15mm, a powder-mixed or two-part filler gives a stronger, harder result, and powder filler is also the better choice anywhere that will take a knock, such as behind a door or below a light switch.

Wide, shallow dips and undulations are the hardest to hide. Use a broad filling knife and pull the filler across the hollow in long, confident strokes, then feather the edges out well beyond the actual dip so the repair blends into the surrounding wall. Look at it in raking light before you sand.

Sanding and Dust Removal

Once the filler is properly dry — usually 30 minutes for lightweight products, longer for powder fillers — sand it back. Start with 120 grit on a block, keeping the block flat so you do not dish the surface, then finish with 180 or 240 grit for a smooth edge. Wrap a sanding sponge around any corners so you do not sand a flat spot into them.

The key is to sand just enough. You are aiming for the repair to be flush with the wall, not to remove the surrounding texture. Run your fingertips over the edge; if you can feel a ridge, keep going gently. When you are happy, wipe the whole area down with a tack cloth and hoover the floor and skirting before you bring any paint into the room. Dust left on the wall will show up as gritty lumps in the finish.

Priming the Repairs

Filler is far more absorbent than the plaster or paint around it, which is why patches so often dry as dull, flat blotches that flash through the topcoat. Seal every filled area before you paint properly. The simplest fix is a mist coat: dilute your emulsion with around 10 to 20 per cent water and brush it over the repairs, allowing it to soak in and dry.

If you are painting over previous dark colours, tobacco-stained walls or water marks, use a stain-blocking primer on those areas first. Two thin coats of emulsion will not cover a stubborn mark on their own, and no amount of extra paint will stop it bleeding through.

Applying Two Thin Coats of Emulsion

Cut in first. Use a brush to paint a neat band along the ceiling line, corners, skirting and around sockets, keeping a wet edge so the roller can blend into it. Then roll the main field of the wall in a large W pattern and fill it in without lifting the roller, working in sections roughly a metre square.

Thin coats always beat thick ones. A heavy coat dries unevenly, sags, and takes far longer than the tin suggests. Two thin, even coats give you better coverage, a more uniform sheen and a finish that lasts. Most vinyl matt emulsions are recoatable in two to four hours, but check the tin and be patient — painting over tacky emulsion leaves roller marks that you cannot remove afterwards.

Finish each section with light, parallel strokes, often called laying off, from skirting to ceiling in one continuous movement. This flattens the stipple and leaves a far smoother surface. Change your roller sleeve between coats if it starts to clog, and keep a damp cloth handy for the inevitable drips.

Details That Make the Difference

Fill and caulk the joint between skirting boards and walls before you paint the walls — decorator's caulk takes paint beautifully and hides an untidy line. If you are painting the ceiling too, do it first and let it dry, then wipe any splashes before they set. Remove your masking tape while the final coat is still slightly tacky, pulling it back on itself at a low angle. Leave the room to dry fully for a day before moving furniture back, and keep the heating low so the paint cures evenly rather than drying too fast and cracking.