Filling Cracks and Holes Before Painting Walls

Why the Prep Work Decides the Finish

A flawless paint job almost never comes down to the paint itself. It comes down to what sits underneath. Cracks, dents, old screw holes and crumbling plaster will all telegraph through even the most expensive emulsion, and no amount of extra coats will hide them. Worse, they often show up as dull or shiny patches, a problem known as flashing, which makes a freshly decorated room look tired before you have even put the brushes away.

The good news is that filling and making good is genuinely one of the easier DIY skills to learn. It costs very little, needs only a handful of tools, and the difference it makes is enormous. Give yourself a couple of hours before you paint and your walls will thank you for years.

Work Out What You Are Actually Filling

Before you reach for the filler, spend five minutes assessing the damage. Different problems need different products, and using the wrong one is the most common reason repairs reappear within a season.

• Hairline cracks in plaster — often caused by shrinkage as the plaster dries, or by slight movement in the building.
• Wider, straight cracks — frequently along joints between plasterboard sheets or where wall meets ceiling.
• Screw and nail holes — usually shallow and simple to fill.
• Dents and gouges — from furniture, door handles or removed fixings.
• Crumbling or blown plaster — where the surface moves or sounds hollow when tapped.

That last one matters. If plaster is hollow or soft, filler will simply crack away with it. Scrape back anything loose, and if the area is larger than a saucer, it is worth treating with a stabilising primer or getting a plasterer to look at it. Similarly, if a crack keeps returning after filling, or the wall feels damp, stop and investigate the cause first. Painting over a damp problem only hides it for a few weeks.

Choosing the Right Filler

There is no single perfect filler, so keep two or three in your kit.

• Powder filler — mixed with water to a smooth paste. Excellent value, dries hard, and ideal for deeper holes. Mix small amounts, as it sets quickly.
• Ready-mixed lightweight filler — the everyday workhorse. Fine-grained, easy to sand, and perfect for hairline cracks, dents and screw holes.
• Flexible filler or decorator's caulk — for joints that move, such as where skirting meets wall or around door frames. It flexes with the building instead of cracking.
• Two-part filler — a chemical-setting product for deep holes, rotten timber repairs and areas that need to harden fast.

For fine, stable cracks, a fine surface filler or even a decent-quality decorator's caulk worked in with a filling knife will do the job neatly. Keep the tube nozzle clean and you will avoid a messy smear across the wall.

Filling Properly, Step by Step

Start by removing dust and loose material. A quick brush down, followed by a wipe with a barely damp cloth, gives the filler something to grip. Let the surface dry completely.

• For small holes, apply filler with a flexible filling knife, pressing it in firmly and slightly overfilling.
• For cracks, open them very slightly with the corner of your knife so the filler can key in, then work it across the crack at a shallow angle.
• For deep holes, build up in two or three thin layers rather than one thick dollop. Thick filler shrinks, sags and takes forever to dry.
• Scrape the surface flush with the wall while the filler is still soft, using the knife at a low angle.

Now leave it alone. Drying times vary from twenty minutes for a thin skim to several hours for deeper fills, and cool or humid rooms slow things down considerably. Filler that looks dry on the surface can still be soft underneath, and sanding too early will drag it out of the hole.

Sanding Flush Without Gouging the Wall

Once fully dry, sand the repair flush with the surrounding plaster. Use 120-grit abrasive paper wrapped around a cork or foam block for flat areas, and finer 180-grit for a smooth finish. Keep the block flat against the wall and work in light, circular passes, checking with your fingertips as you go. Your hand will feel a ridge long before your eye sees one.

Feather the edges of the filler so there is no hard lip where it meets the original wall. When you are happy, dust everything down thoroughly. Filler dust is fine, chalky and loves to cling to walls, so give the surface a wipe with a damp cloth and let it dry. Any dust left behind will show through paint as a gritty texture.

Spot-Priming to Prevent Flashing

This is the step most people skip, and it is the one that gives the game away. Filler is far more porous than plaster or old paint, so it drinks in emulsion and dries to a different sheen. The result is a patchwork of dull spots that catch the light, even under two full coats.

The fix is simple: spot-prime every repair before you paint the wall. A thinned coat of the same emulsion works well, or use a dedicated primer-sealer for a more reliable result. Apply it with a small brush or mini roller, extending slightly beyond the repair, and let it dry fully. On bare plaster patches, a mist coat — emulsion thinned with around ten to twenty per cent water — seals the surface and stops the paint from being absorbed unevenly.

Once primed, your repairs should vanish under the topcoats. Paint the whole wall rather than just the patch, cutting in neatly at ceilings, skirting and corners. Two full coats of a quality emulsion, applied in consistent strokes, will give you the even, professional finish you were after — and one that will hold up far better than a rushed job ever could.