Why has my silicone not cured or not stuck?

Silicone rarely fails at random. Each failure mode points at a specific cause, and all of them are decided before the gun is even picked up.

What you'll need

• fresh silicone sealant
• isopropanol or methylated spirit
• sealant removal tool or blade
• masking tape (optional)
• smoothing tool (optional)

Step by step

1. Identify which failure you have.Permanently soft and sticky is a curing failure. Cured but lifting away in a strip is adhesion. Cured and split along its length is joint movement.
2. Check the date on the tube.Silicone has a shelf life, typically twelve to eighteen months, and old stock cures slowly or not at all. This is the most common cause of a bead that never hardens.
3. Check the temperature and ventilation.Most silicone cures by reacting with atmospheric moisture and needs to be above about five degrees. A cold unventilated bathroom in January cures very slowly.
4. Remove every trace of the old sealant.The invisible silicone film left behind after the bulk is cut away is what new silicone refuses to stick to. Scrape, then wipe with a proprietary residue remover or isopropanol.
5. Degrease and dry the surfaces completely.Soap film, limescale, cleaning product residue and body oils all prevent adhesion. Wipe with isopropanol and let it flash off.
6. Check for a mould-resistant product where it matters.In a shower, a sanitary-grade silicone with a fungicide is worth the small extra. Ordinary silicone blackens quickly in constant damp.
7. Avoid three-sided adhesion.The bead should bond to two faces only. If the gap is deep, fit a backing rod or a strip of closed-cell foam so the bead cannot grip the bottom.
8. Cut the nozzle to suit the gap and apply in one pass.A steady continuous bead with even gun pressure. Restarting leaves a weak point that splits first.
9. Tool the bead promptly.A smoothing tool or a soapy finger, once, firmly. Tooling presses the silicone into both faces, which is what creates the bond, and forms the concave profile that can stretch.
10. Load the joint before sealing it where relevant.Fill a bath with water before sealing round it, and leave it full until the silicone has cured. The bath then drops no further, so the joint is sealed at its worst-case position.
11. Keep it dry for the full cure.Twenty-four hours minimum for a normal bead, longer in cold or deep sections. Using the shower early is a common cause of a bead that lifts within days.

Tips

• Filling the bath before sealing it is the trick that stops the bead tearing the first time someone gets in.
• Silicone will not stick to silicone. Removing the invisible residue is the whole battle when resealing.
• Check the tube's date. Half of all curing failures are simply old stock.

Common mistakes

• Applying new silicone over old residue — Cured silicone is a release surface. Even an invisible film prevents the new bead bonding, so it peels off in one piece within weeks.
• Filling a deep gap solid with silicone — The bead bonds to three faces and cannot stretch, so the first movement splits it lengthways. It also never cures properly in the middle.

If it doesn't work

Questions people ask