Fill gaps around a window reveal inside

Fill and seal gaps around an inside window reveal, choosing the right filler and technique for your wall type.

WR-01Prepare the roomMove curtains and blinds out of the way and lay a dust sheet under the window. Open the window if you can, for ventilation. Wipe the reveal down with a damp cloth so you can see the gaps clearly.

1 each · Dust sheet

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1 roll · Masking tape

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WR-02Work out what's behind the revealKnock gently along the reveal. Solid brick or block sounds dense and the same all the way along. Plasterboard on studs or dot-and-dab sounds hollow, and on dot-and-dab the board can sit up to 40mm off the brick behind, so deep gaps here have a real void behind them, not just a crack. Lath and plaster reveals (common pre-1930s) feel slightly springy and old cracked patches often crumble to a powder when pressed.

WR-03Rake out old filler and loose materialUse a filling knife or scraper to remove any cracked old filler, sealant or loose plaster. Dig out anything that isn't firmly stuck rather than filling over it. Vacuum the gaps and reveal so the new filler bonds properly.

1 each · Filling knife

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WR-04Fill deep gaps (over 5mm) firstDon't pack deep gaps with filler alone, it will shrink and crack. On solid brick or block, low-expansion foam sealant can go straight against the masonry. On dot-and-dab or a stud wall, push a foam backer rod into the gap first to fill most of the depth, then top with filler or foam so you're not relying on foam to bridge a large void. On lath and plaster, skip high-expansion foam, as it can press on the laths and crack the old plaster further; use backer rod and a flexible filler instead.

1 can · Low-expansion foam sealant

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1 pack · Foam backer rod

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WR-05Fill shallow cracks and holesFor gaps under 5mm, press a general-purpose filler in with a filling knife, slightly overfilling since it shrinks a little as it dries. On lath and plaster, use a flexible or lime-compatible filler rather than a rigid gypsum one, as ordinary filler can crack away from old lime plaster. Leave to dry fully before the next step, usually a few hours.

1 tub · Multi-purpose filler

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WR-06Sand smoothOnce dry, sand the filled areas flush with a fine sanding sponge. If the window frame paint is flaking and the property is older, it may contain lead; wipe dust away with a damp cloth rather than sanding it aggressively.

1 each · Fine sanding sponge

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WR-07Seal the joint between frame and wallThe line where the window frame meets the plaster or masonry moves slightly with temperature, so use a flexible decorator's caulk here, not rigid filler, on every wall type. Run a neat, continuous bead with a sealant gun and smooth it with a wet finger or a caulking tool before it skins over.

1 tube · Flexible decorator's caulk

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1 each · Sealant gun

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WR-08Prime and touch up paintOnce the filler and caulk are fully dry, dab on a spot of primer over the repaired areas before painting, so the finish colour doesn't look patchy. Apply matching paint once the primer is dry.

1 small tin · Multi-surface primer

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1 small tin · Matching paint

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WR-09Tidy upFold the dust sheet inward to trap dust and dispose of scraped-off waste. Wipe the sealant gun and filling knife clean before the caulk sets on them. Wash brushes and let the room air.

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