Why Preparation Decides the Finish

Most disappointing paint jobs fail before the first brushstroke. Paint is a thin film, usually only 30 to 50 microns thick, so it faithfully copies whatever surface sits beneath it. Every ridge, crater and hairline crack shows up in raking light once the room is finished. Sanding and filling take time, but they are the cheapest part of the job and the one that separates a professional-looking room from a tired first attempt.

Start by clearing the room as far as you can. Move furniture out, take down curtains and pictures, and lay dust sheets over the floor. Good light is essential: fit a bright work lamp at a low angle so it skims across the wall and throws shadows into every imperfection. Ordinary ceiling light flattens texture and hides the very faults you need to find.

Assessing the Wall and Choosing Your Filler

Run a clean hand across the plaster. You are feeling for three things: movement, hollows and grit. Any area that sounds hollow when tapped or flexes under light pressure needs attention first, because filler will simply crack again. Small hollow patches can often be stabilised with a penetrating primer, but large areas of blown plaster are a job for a plasterer.

Wash greasy walls with sugar soap and warm water, then rinse and let them dry completely. Kitchens and hallways usually need this; bedrooms often do not. Gloss or silk finishes from previous decorating should be dulled with a fine abrasive so new paint can grip.

• Fine surface filler for hairline cracks, dents and shallow dings. It dries fast and sands to a feather edge.
• Powder filler mixed with water for deeper holes, chases and sections of missing plaster. It sets hard and takes longer to sand.
• Decorator's caulk for the joints where wall meets skirting, architrave or ceiling, where movement will always reopen a rigid filler.
• Scrim tape and a wide filling knife if a crack keeps returning.

Filling Cracks and Holes Properly

A crack is rarely just a crack. Open it out gently with a filling knife or the corner of a scraper so the edges are sound and the slit is wider than it is deep. Brush out the dust, then press filler in firmly across the line rather than along it, so it is forced into the gap. Draw the knife back at a shallow angle and leave the filler very slightly proud of the surface.

Deep holes should be built up in layers, no more than 5mm at a time, letting each layer set before adding the next. Stuffing a hole in one go traps moisture underneath, and the filler will shrink into a dish shape as it dries. For a stubborn crack that sits along a joint, bed a strip of scrim tape into the first layer while it is still wet, then skim over it once set.

Caulk is the right choice anywhere two surfaces meet and move. Run a thin bead along the top of the skirting, smooth it with a damp finger or a caulking tool, and wipe the excess away immediately. Never use caulk to fill a hole in the face of a wall, as it stays soft and cannot be sanded.

Sanding Filler Flush

Let the filler dry for the full time stated on the tub, then test a corner with a fingernail. If it dents, it is not ready. Wrap abrasive paper around a cork or foam sanding block rather than using your fingers, as fingers create hollows that show badly under paint. Work in light, circular passes, then finish with strokes along the wall so you do not leave visible scratches.

• 120 to 150 grit for knocking down proud filler and blending edges.
• 180 to 240 grit for the final pass on filler and for keying old paint.
• A flexible sanding sponge for curved mouldings and the corner where wall meets ceiling.

The aim is not to remove the filler but to bring it level with the surrounding wall. Feather the edges so the repair fades into the plaster with no detectable lip. Hold the lamp low and check as you go. Once you are happy, brush or vacuum the dust away and wipe the wall with a damp cloth or tack cloth, then let it dry.

Priming for an Even Surface

Primer is what stops the finished paint from looking patchy. Bare filler is far more absorbent than the surrounding plaster and will drink the topcoat, leaving dull, flat patches that no second coat will fully hide. Any repair you have made should be spot-primed before the wall is painted.

Fresh plaster, filler and old stains all need slightly different treatment. Choose a primer that suits the job rather than assuming one tin does everything:

• Bare plaster and filler: a thinned first coat, often called a mist coat, made from emulsion and around 10 to 20 per cent water.
• Water stains, smoke or marker pen: a stain-blocking primer, or the marks will creep back through the finish.
• Previously glossed or silk surfaces: a gripping primer after sanding, so the new paint bonds rather than peels.
• Water-damaged areas: an alkali-resisting primer to neutralise the salts left behind.

Apply primer with a roller for speed and a brush for the edges, working it into corners and any texture. One even coat is usually enough, but absorbent areas may take two. Let it dry thoroughly before you assess the wall again; flaws that survive primer will still be there under the topcoat, and this is your last easy chance to fill and sand them.

Small Habits That Make a Big Difference

Keep your filling knife clean as you work, wiping it on a rag rather than letting dried lumps drag through fresh filler. Never sand in a room without opening a window and wearing a dust mask, and seal the door with a sheet to stop dust travelling through the house. Buy slightly more filler than you think you need, as a second pass is far easier than a trip out mid-job.

Finally, be patient between stages. Filler, caulk and primer all need proper drying time, and rushing them is the single most common reason a wall ends up looking worse than it did before you started. Give each stage its full drying window, sand with care, prime evenly, and the topcoats will reward you with a flat, smooth, evenly coloured surface that looks genuinely professional.