Why did my new bathroom sealant go black within weeks?

A properly applied sanitary silicone bead in a reasonably ventilated bathroom should look clean for years. Going black in weeks is not bad luck and it is not a bad brand — it means one of five specific things happened, and four of the five are about what was underneath the bead rather than the bead itself. Working out which one it was decides whether resealing it again will work.

Step by step

1. Consider the substrate first — this is the usual answer.Old silicone leaves an invisible film, and mould lives in the joint underneath a bead, on the tile edge and on the tray or bath surface. If the joint was not taken back to clean, bare, sterile surface, the new bead is laid straight over a live colony which grows up through and behind it. It appears as blackening from below within weeks.
2. Ask whether the joint was properly dry.Silicone cures by reacting with moisture in the air, so it will cure over a damp joint — but the damp is then sealed in, with a food supply, at bathroom temperature. A bath filled for sealing must have its rim dried; a shower must not be used for the drying period; and a joint that has been leaking must be given real time, sometimes days with a fan, before anything goes into it.
3. Check what sealant was used.General-purpose silicone contains no fungicide. Sanitary or bathroom-grade silicone contains a fungicide that leaches slowly out of the bead and suppresses colonisation for a few years. In a shower, using the wrong tube is a guaranteed early failure. Acrylic sealant in a wet area fails faster still, because it is porous.
4. Look at the shape of the bead.A bead tooled flat, or one that leaves a small ledge, holds a film of water after every use. A concave bead tooled properly sheds water. Standing water on a sealant joint is the single environmental factor that most reliably produces mould, and it is entirely a matter of how the bead was finished.
5. Look at whether the joint is moving.A bath or tray that flexes stretches the bead every time it is used. Microscopic tears open along the edges, water gets behind, and mould grows in the gap and shows through. If the bath moves when you get in it, that is the cause and no amount of resealing addresses it.
6. Then look at the room.A bathroom that is still visibly wet an hour after a shower will grow mould on anything. Extraction during and after the shower, the door shut while the fan runs, the screen and tiles squeegeed, and towels dried elsewhere. This is the difference between a bead lasting five years and lasting five weeks.
7. Decide whether to reseal now or fix the cause first.If the cause was a contaminated or damp substrate, redoing it properly will work. If the cause is a moving bath or a room that never dries, reseal after fixing that — otherwise you are booking the same job again.
8. Do it properly the second time.Cut both edges, peel the bead away, remove every trace of residue with a solvent silicone remover, treat the joint with a mould remover and let it work, rinse, dry completely, wipe with methylated spirit, and only then seal — with sanitary silicone, in one continuous pass, tooled concave once, and left the full cure time before any water touches it.

Common mistakes

• Sealing over the old bead rather than removing it — New silicone does not bond to old, the old bead's mould is still alive underneath, and the joint is now twice as thick and twice as likely to hold water.
• Using general-purpose silicone in a bathroom — No fungicide. It will colonise within months in a shower and there is nothing wrong with it otherwise.
• Tooling the bead flat because it looks tidier — A flat bead holds a film of water after every shower. Concave sheds it.
• Resealing a bath that still moves when you get in it — The bead is stretched every use, tears at the edges, and water gets in behind it. Fix the support first.

Questions people ask