Seal a leaking uPVC window frame joint

Strip out old, failed sealant around the window frame and replace it with a neat, weatherproof bead to stop draughts and leaks.

SW-01Check the weather and find the leakLook closely at the joint between the uPVC frame and the wall, and at the corner joints of the frame itself, to see where the old sealant has cracked, shrunk or pulled away. Check the small weep holes along the bottom of the frame are not blocked, as blocked weep holes can cause leaks that look like a sealant problem. Pick a dry day above 5°C with no rain forecast for at least 24 hours, since sealant needs to cure properly.

SW-02Gather your tools and materialsGet everything ready before you start so you are not stopping halfway through a wet bead of sealant. You need a sealant remover tool, a sealant gun, low-modulus silicone sealant made for uPVC, isopropyl alcohol, a lint-free cloth, low-tack masking tape and gloves.

1 each · Sealant remover tool

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1 cartridge · Low-modulus silicone sealant (300ml, weatherproof, for uPVC)

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1 each · Skeleton sealant gun

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1 bottle · Isopropyl alcohol

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1 each · Lint-free cloth

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1 roll · Low-tack masking tape

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1 pack · Disposable gloves

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SW-03Strip out the old sealantRun the sealant remover tool along both edges of the failed joint to cut the old bead free, then peel it out in one piece where you can. Work slowly near the glass and the uPVC surface so you do not scratch either. Pick out any stubborn bits left behind with the corner of the tool.

SW-04Clean and degrease the jointWipe the whole joint with a cloth dampened in isopropyl alcohol to remove dust, old silicone residue and any mould. Sealant will not stick properly to a dirty or greasy surface, so go over it twice if needed. Leave the joint to dry fully for at least 30 minutes before you carry on.

SW-05Mask the edgesStick a strip of low-tack masking tape along each side of the joint, leaving a gap of around 5-6mm between the two strips for the new bead. Press the tape down firmly so no sealant can creep underneath. This gives you a straight, professional-looking edge with no scraping needed afterwards.

SW-06Load the gun and cut the nozzleCut the tip of the sealant cartridge nozzle at a 45° angle so the opening roughly matches the width of the gap you have masked. Pierce the foil seal inside the nozzle with a long thin tool or a bent wire. Load the cartridge into the sealant gun.

SW-07Apply the new sealantHold the gun at a steady 45° angle and squeeze the trigger evenly as you move along the joint in one continuous pass. Keep the same speed and pressure so the bead comes out an even thickness. Try not to stop and start partway along, as this leaves a visible join in the bead.

SW-08Tool and smooth the beadDip your finger or a smoothing tool in water with a little washing-up liquid, then run it along the bead in one smooth pass to press the sealant into the gap and give it a neat, slightly concave finish. Do not go over it more than once, as this drags the sealant and spoils the finish.

1 each · Sealant smoothing tool

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SW-09Remove the masking tapePeel away both strips of masking tape straight away, while the sealant is still wet, pulling each strip back on itself at a 45° angle. Waiting until the sealant has started to skin over will tear the edge of the bead.

SW-10Leave to cure and tidy upLeave the sealant undisturbed for at least 24 hours, and check the cartridge label for the full cure time, before opening the window, touching the joint or letting rain hit it. Wipe the sealant gun clean and wash the smoothing tool. Put the used nozzle and old sealant scraps in the bin, not down the sink.

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