Finishing window sills and reveals after replacing windows

What usually gets left unfinished after new windows go in

Most window fitters cut the foam flush, check the frame is sealed and level, and move on. What they leave behind is the reveal — the plastered surface around the frame — often chipped from the old window's removal, with a ragged line where old and new plaster meet.

The sill is a separate item entirely. Many quotes for window replacement do not include a new sill, inside or out, so you may be looking at bare brick, an old sill that no longer fits, or a gap where one should sit.

Then there is the join between frame and wall: expanding foam is not a finish. It degrades in sunlight and needs covering, and the gap between frame and plaster needs a proper seal, not just foam showing through a skim of paint.

Tools and materials you will need

Most of this is a small job in terms of materials, but it is worth buying the right product rather than improvising. For interior reveals you want a ready-mixed finishing plaster or filler, a small trowel or filling knife, sandpaper (120 and 240 grit), and a sanding block.

For the sill, measure the reveal width and the overhang you want before buying — a PVC or MDF window board for interior sills, or a proper stone or metal sill for the exterior, cut to length or ordered to size. You will also need silicone sealant rated for the location, a sealant gun, masking tape, and a damp cloth for tooling the bead.

A stanley knife for trimming excess foam, a spirit level, and a mitre box if the reveal has corners to finish neatly complete the list. Buy slightly more filler and sealant than you think you need — running out mid-job on a Sunday is a common frustration.

• Ready-mixed finishing plaster or interior filler
• Filling knife or small trowel, sanding block, 120 and 240 grit paper
• Interior window board (PVC or MDF) and, if needed, an exterior sill
• Silicone sealant suited to indoor or outdoor use, sealant gun
• Masking tape, damp cloth, stanley knife, spirit level

Plastering and levelling the reveals, step by step

Start by trimming any foam that stands proud of the wall with a sharp knife, cutting it back roughly level with the surrounding plaster. Brush away dust and loose debris, and dampen the old plaster edge slightly so the new filler bonds rather than drying too fast and cracking.

Apply the filler in thin layers rather than one thick pass — a thick fill is more likely to shrink and crack as it dries, especially in the corners of the reveal where movement is greatest. Let each layer go off before adding the next, following the drying time on the tub.

Once the final layer is proud of the surface, sand back with the 120 grit paper first, then 240 to finish, checking with a straight edge or the side of your hand for flat spots. In Soviet-era panel flats the reveal is often not quite square to begin with, so aim for a straight, even line rather than a perfect right angle.

The reveal is ready for paint once it is smooth, dry and dust-free — our painting and decorating service, from 12 € per job, covers just this kind of small finishing area if you would rather not do the cutting-in yourself.

Fitting the sill: what to get right

The interior sill should sit level, with a slight fall is not needed indoors, but it must be firmly supported along its whole length — a sill that flexes in the middle will crack any sealant joint above it within a season. Pack the gap underneath with expanding foam or mortar before fixing, not left hollow.

The exterior sill is more critical: it must slope away from the building, typically enough that water runs off rather than pooling against the frame, and it should overhang the wall face so rain drips clear rather than running down the render. Getting this fall wrong is one of the more expensive mistakes to fix later, since it can let water track back into the reveal.

Cut sills to length with a fine-tooth saw for PVC or MDF board, always leaving a small gap at each end for the sealant joint rather than cutting tight to the wall. If your windows were fitted as part of a larger job, ask whether sill fitting was included — it often is not, and our window fitting and servicing service, from 90 € per job, can supply and fit one to match.

Sealing the joints and the final check

Silicone goes on last, after plaster and paint are fully dry and the sill is fixed. Mask both sides of the joint with tape for a straight line, apply a continuous bead with the gun held at a steady angle, then tool it smoothly with a damp finger or a proper sealant tool before the skin forms.

Seal the joint between frame and reveal on both inside and outside, and the joint between sill and wall underneath. Leave expansion gaps at the ends of the sill rather than filling them solid with sealant, which stops the sill from moving slightly with temperature without cracking the seal.

Once everything is dry, run your hand along each joint and check for gaps, and open and close the window to confirm nothing binds against fresh sealant. Our silicone sealing service, from 45 € per job, is worth considering if you want a clean, even bead without redoing it twice — it is a fiddly job to get looking neat on a first attempt.

When it makes more sense to call a tradesman

A small reveal with minor plaster damage and a straightforward interior sill is a sound DIY job for anyone comfortable with basic filling and sanding. Where it gets harder is a wide reveal with structural damage, a sill that needs cutting to an unusual angle, or a wall where the old sill was mortared in and removing it damages the surrounding brick.

If the gap between frame and wall is large — more than the foam can bridge properly — or if there is any sign of damp or rot in the reveal itself, that points to a problem with the original installation rather than a finishing job, and is worth having looked at properly rather than plastered over.

We do not touch load-bearing walls, so if enlarging a reveal or altering an opening is part of the plan, that needs a structural specialist first. For everything short of that — sill supply and fitting, sealing, or the finishing plaster and paint — it is often quicker to have it done properly in a single visit than to buy tools for a job you will do once.