Fillers: choosing and using them

Learn to choose the right filler and apply it so cracks and holes disappear invisibly.

FI-01Know your filler typesReady-mixed filler comes in a tub, is soft and easy to spread, and suits small marks and nail holes. Powder filler comes as a dry mix you add water to yourself; it sets harder and is better for anything you'll sand or paint over. Decorator's caulk comes in a tube for a sealant gun and is flexible, used for gaps that move slightly, like where skirting meets the wall. The beginner mistake is grabbing whatever's under the sink and using it for everything.

FI-02Match the filler to the jobFor small nail holes and hairline cracks, use ready-mixed multi-purpose filler. For deep holes (anything past about 10mm deep) use a deep-hole or 'one coat' filler, because ordinary filler shrinks and cracks if you pack it in thick. For gaps that flex, like skirting boards or where two different materials meet, use decorator's caulk, not rigid filler, or it will crack again within weeks.

1 tub (500g) · Multi-purpose ready-mixed filler

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1 tub (600ml) · Deep-hole/one coat filler

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1 tube (310ml) · Decorator's caulk

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FI-03Prepare the hole or crack firstUse a filling knife (a flexible flat metal blade, a bit like a small trowel) or the corner of a scraper to knock away any loose or crumbling material from the hole. Blow or brush out the dust, because filler won't stick properly to a dusty surface. The common mistake is filling straight over loose bits, which means the patch falls out later.

FI-04Load the filling knife correctlyScoop a small blob of filler onto the tip of your filling knife, not the whole blade. Press it into the hole at an angle, working the filler into the gap rather than smearing it across the surface. Too little filler on the knife and you'll just push air around; too much and it goes everywhere except the hole.

FI-05Draw the knife across to level itOnce the hole is packed, hold the filling knife almost flat against the wall and draw it across the filler in one smooth pass, slightly overfilling so it sits just proud of the surface. Wipe the blade clean between passes. Multiple scrappy strokes leave ridges that are much harder to sand flat later than one confident pass.

FI-06Build deep holes in layersFor anything deeper than about 10mm, fill in stages rather than one thick blob. Apply a layer, let it go touch-dry (check the tub for timing, usually a few hours), then add the next layer on top. Filling too deep in one go is the number one reason filler cracks or sinks as it dries.

FI-07Sand back once fully dryWait until the filler is completely dry and hard, not just dry to the touch; check the tub for the full cure time. Sand gently with fine-grit sandpaper (around 120-180 grit) wrapped around a small block, using light circular motions until the patch is flush with the surrounding wall. Pressing too hard or sanding too soon will gouge a dip into the soft filler.

1 sheet · Fine-grit sandpaper (120-180 grit)

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FI-08Check you've got it rightRun your open hand flat across the patch with your eyes closed; you should feel no ridge or dip, just smooth wall. Then look across the surface at a low angle with a light behind you, which shows up any bumps or hollows that your hand missed. If it's still visible under this light test, add a thin skim coat and sand again rather than painting over a flaw, because paint makes imperfections more obvious, not less.

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