How do I fix cracked or mouldy sealant?

Sealant fails in two ways: it cracks and lets water behind, or mould grows into it. Both mean replacement. The single thing that determines whether the new bead lasts is how completely you remove the old one, because new silicone will not bond to old silicone.

Safety

What you'll need

• Sanitary silicone and a cartridge gun
• Sealant removal tool or sharp knife
• Silicone remover solvent
• Masking tape
• Isopropyl alcohol
• Cloths

Step by step

1. Decide whether it needs replacing: cracked, lifting, or black through the body of the silicone all mean yes.Surface mould on otherwise sound sealant can be cleaned. Mould inside it cannot.
2. Cut along both edges of the bead and lift it away in strips.Work along the joint. Long strips come out cleanly; picking leaves fragments.
3. Remove every remaining trace with a solvent silicone remover and a plastic scraper.This is the whole job. New silicone will not bond to old silicone, so any residue is a guaranteed failure point.
4. Clean the joint with isopropyl alcohol and let it dry completely.Any moisture, soap film or dust becomes a failure point.
5. If sealing around a bath, fill it with water first.A full bath sits several millimetres lower. Sealing empty means the bead tears the first time it is used.
6. Mask both sides of the joint, leaving the gap you want the bead to be.This is what gives a straight professional line, and it takes ten minutes.
7. Cut the nozzle at 45° and apply one continuous bead with steady pressure.Stopping and restarting shows. Cut the nozzle smaller than you think — you can widen it, not narrow it.
8. Tool the bead smooth in one pass, then pull the tape off immediately while the silicone is wet.Waiting until it skins drags a ragged edge into the bead.
9. Leave it to cure for 24 hours before draining the bath or using the shower.Check the tube. Cold, dry air slows curing considerably.

Tips

• Use sanitary-grade silicone with a mould inhibitor. Standard silicone in a wet area grows black spots within a year.
• A drop of washing-up liquid in water makes a good tooling solution and stops the silicone sticking to your finger.
• Warm the cartridge in a bowl of warm water in cold weather. Cold silicone is very hard to extrude smoothly.

Common mistakes

• Sealing over old silicone — It will not bond. The joint fails within weeks and you do the job twice.
• Sealing an empty bath — The bath drops when filled and the new bead splits immediately.
• Leaving masking tape on until the silicone skins — Pulling it then tears a ragged edge into the bead.

Questions people ask