How do I reseal around a window?

The joint between a window frame and the wall moves with temperature and takes direct weather. That is why it fails, and why it needs a sealant that can stretch rather than the cheapest tube on the shelf.

Safety

Step by step

1. Work out which joint has failed.The external perimeter between frame and masonry, the internal joint between frame and plaster, or the glazing seal within the frame itself. They need different products and only the first two are a resealing job.
2. Remove the old sealant completely.Score both edges with a sharp knife, pull the bead out in one length if you can, then scrape the residue off with a plastic scraper or a sealant removal tool. This is most of the work and it is the step that decides whether the job lasts.
3. Clean and dry the joint thoroughly.Brush out dust, then wipe with a solvent-based cleaner and let it dry completely. Sealant will not bond to dust, old silicone residue or a damp surface.
4. Choose the right sealant for outside.A hybrid MS polymer for external joints — it bonds to damp surfaces, tolerates movement, resists ultraviolet and takes paint. Not decorator's caulk, which shrinks and cracks outdoors within a year.
5. Use silicone only where nothing needs painting.It seals well and cannot be painted at all. On a frame within a run of paintwork it is the wrong choice regardless of its other properties.
6. Fill deep gaps with backer rod first.A gap deeper than it is wide cures badly and wastes sealant. Foam backer rod pressed in sets the depth so the bead bonds to two sides and can stretch properly.
7. Mask both sides for a crisp line.Tape on frame and masonry leaving the gap you want. Five extra minutes, and it is the difference between a first attempt and a neat one.
8. Cut the nozzle small and at an angle.45 degrees, starting smaller than the gap — you cannot uncut it. The angled face lies along the joint and shapes the bead as it comes out.
9. Push the gun, do not pull it.Pushing forces sealant into the joint ahead of the nozzle. Pulling drags it across the surface, which looks acceptable and adheres poorly.
10. Tool the bead within two minutes.A wetted finger or a shaping tool, using soapy water rather than plain. Tooling presses it into contact with both faces — an untooled bead frequently is not actually stuck.
11. Pull the tape while it is still wet.Immediately after tooling, away from the bead at a shallow angle. Left to skin over, the tape tears the edge off with it.
12. Leave the weep holes clear.The small slots along the bottom of a UPVC frame drain water out of the frame. Sealing over them traps water inside, which is worse than the leak you were fixing.
13. Check the cure time against the forecast.Most need several hours dry, some considerably longer. Doing an external joint the afternoon before rain wastes the whole job.

Tips

• Buy one more tube than you think. Running out mid-joint means a visible join and a part-cured start.
• If the gap is more than about 10mm, the joint may need filling or repointing rather than sealing. Sealant is not a filler.
• Do the internal joint with decorator's caulk and the external one with MS polymer. They are different jobs with different products.

Common mistakes

• Sealing over the old bead — The new sealant bonds to the old one rather than the joint, so the whole lot lifts away as a strip at the first movement.
• Sealing over the weep holes — They drain water out of the frame. Blocking them traps water inside the frame, which causes worse damage than the draught did.

If it doesn't work

Questions people ask