Strip old cracked sealant, clean the gap, then run a fresh bead of sealant around the window frame to stop draughts and damp.
SW-01Clear the area and protect surfacesMove curtains, blinds and anything on the windowsill out of the way. Lay a dust sheet below the window to catch scrapings. Open the window fully if you can, or another window nearby, so you're not breathing in fumes from old sealant or fresh product.
SW-02Cut out the old sealantRun a sharp knife down both edges of the old bead, then lever it out with a filler knife or scraper. On a solid brick or block reveal this is straightforward. On a lath and plaster reveal, work slowly and don't dig the scraper in — old lime plaster behind can crack or crumble if you push too hard. On a dot-and-dab plasterboard reveal, keep the scraper shallow; there's a void behind the board and gouging too deep can punch through it.
1 each · Sharp utility knife 1 each · Filler knife / scraper SW-03Clean and dry the gapWipe out dust and old sealant residue with a cloth dampened with methylated spirit or mild detergent. Let the gap dry fully before you continue — sealant won't grip to a damp or dusty surface. Check for any soft, damp or blackened plaster as you clean; that points to a leak or condensation problem that sealant alone won't fix.
1 bottle (500ml) · Methylated spirit SW-04Check the gap widthA gap up to about 5mm is fine for sealant on its own. If it's wider than that anywhere — common where a timber frame has shrunk, or around an older lath and plaster reveal — push a foam backer rod into the gap first. This stops the sealant sinking in and gives you a neat, even bead on top.
1 pack · Foam backer rod (6mm) SW-05Mask the edges for a neat lineRun a strip of masking tape along the frame edge and another along the wall edge, leaving only the gap exposed. This is optional but makes a much straighter, tidier finish, especially on plaster or painted plasterboard where a wobbly line shows.
SW-06Load the gun and apply the beadCut the sealant tube nozzle to match the gap width and load it into the gun. Squeeze a steady, even bead along the full length of the gap in one continuous pass, holding the gun at a consistent angle. Don't stop and start halfway along a run if you can help it — it's harder to blend.
1 tube (300ml) · Sealant (silicone or decorators' caulk) SW-07Tool the bead smooth and remove the tapeSmooth the bead with a wet finger or a plastic smoothing tool, using one continuous pass. Peel off the masking tape straight away, while the sealant is still wet, pulling it away at an angle so you don't drag the bead.
1 each · Sealant smoothing tool SW-08Leave to cure and ventilateLeave the window undisturbed while it cures — check the tube for the exact time, but allow at least 24 hours before cleaning the window or letting condensation run over it. Keep the room ventilated while it cures, but don't blast the sealant with direct heat from a radiator to speed it up. If the frame has trickle vents, make sure your sealant hasn't covered them — they need to stay clear for background ventilation.
SW-09Clean upWipe the gun nozzle and cap the tube tightly for next time. Fold the dust sheet inward to trap any scrapings and bin the offcuts of old sealant. Wash the smoothing tool and cloths before the sealant on them sets hard.
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