Why Preparation Makes or Breaks a Paint Finish

It is tempting to think a fresh coat of paint will hide all manner of sins. In practice, the opposite is true. Paint is a magnifying glass for every crack, dent and lumpy patch beneath it, and a glossy finish will show up imperfections that a matt one might disguise. Spending an extra hour or two on filling and sanding before you open a tin will save you far more time later, and it will give you a wall that looks properly decorated rather than merely covered.

The good news is that this is genuinely beginner-friendly work. You do not need specialist skills, just a little patience, the right filler and a willingness to sand a bit more than you think is necessary. Work through the stages below and you will end up with a surface that takes paint beautifully.

Gathering the Right Kit

Before you start, lay everything out so you are not hunting for a sanding block with filler setting on your hands. For most jobs in a typical British home, you will need:

• Powdered or ready-mixed filler — powdered is cheaper and sets harder, ready-mixed is more convenient for small jobs.
• Fine surface filler for hairline cracks and shallow imperfections.
• A filling knife or flexible scraper — a 75mm blade suits most work, with a wider one for big areas.
• Sanding blocks or a sanding sponge in medium (around 120 grit) and fine (around 180 to 240 grit).
• A dust sheet, a bucket of warm soapy water and a clean cloth.
• Filler primer or a mist coat of emulsion for sealing afterwards.

If you have old lath-and-plaster walls, keep the filler slightly flexible rather than rock hard, as these walls move a little with the seasons. On modern plasterboard, standard filler is fine.

Filling Cracks and Holes

Start by raking out the crack or hole with the corner of your filling knife. Loose material and crumbling edges need to come away, otherwise your filler will simply sit on top and flake off within weeks. Run the knife along the crack to widen it slightly into a shallow V shape, then brush out the dust.

For hairline cracks up to about 2mm, a fine surface filler applied thinly with a flexible blade is usually enough. Press it in firmly, then scrape the excess flush in one smooth stroke. For deeper holes — old rawlplug holes, nail heads, chunks of missing plaster — build the filler up in two or three thin layers rather than one thick dollop. A thick layer shrinks as it dries and leaves a dip you will have to fill again.

Allow each layer to set fully. Powdered filler typically takes 20 to 30 minutes; ready-mixed can take longer in a cold room. Once set, run your fingertip over the repair. You should feel a slight proudness rather than a hollow. If it feels hollow, add another thin layer.

Sanding Back to a Smooth Finish

When the filler is properly dry, sand the patch back. Use a medium grit on a block for large areas and switch to fine grit for the final pass. Always sand in light, circular motions with very little pressure, keeping the block flat against the wall — if you tip it on edge, you will carve a groove that no amount of paint will hide.

Feather the edges of the repair into the surrounding wall. The aim is to be unable to feel where the filled area ends. Run the flat of your hand across it with your eyes closed; your fingers are far more sensitive than your eyes at this stage.

If you are sanding previously painted walls to key the surface before repainting, a light going-over with fine grit is all that is needed. Do not sand through the old paint into bare plaster, as this will leave a thirsty patch that soaks up paint unevenly.

Dusting Down Thoroughly

This is the step most people skip, and it is the one that causes the most trouble. Sanding dust is incredibly fine and clings to walls by static. If you paint over it, your fresh emulsion will develop a gritty, sandpapery texture and may fail to adhere properly.

Wipe down the whole area with a damp cloth, rinsing it frequently in warm water with a drop of washing-up liquid. Follow with a dry cloth. For a really thorough job, vacuum the walls with a brush attachment first, then wipe. Change your water when it turns grey — you are aiming for a genuinely clean, dust-free surface, not just a less dusty one.

Priming and Spot-Sealing

Bare filler is porous and will drink paint, leaving dull patches that show through your topcoat as a slightly different sheen. Seal every repair before you paint the wall properly. You can use a dedicated filler primer, or simply apply a mist coat — emulsion thinned with around 10 to 20 per cent water — over the filled areas and any bare plaster. Let it dry, then apply your proper topcoats.

Take a moment to look along the wall in raking light from a lamp or a torch held close to the surface. This reveals any dips or ridges you missed. A quick touch-up now is far easier than repainting a whole wall later. Once it looks flat and even in that light, you are ready to paint with confidence.