Why does my silicone keep going mouldy, and how do I stop it?

The fungicide in sanitary silicone protects the sealant. It does nothing about what is lying on the surface, and in a shower that is a continuous layer of soap, shampoo and skin oil — an excellent food source kept permanently warm and wet. That is why a brand new sanitary bead goes black in a year in a badly ventilated bathroom and lasts a decade in a well ventilated one.

Safety

Step by step

1. Work out which kind of blackening you have.Diffuse blackening across the face of the bead is surface mould on the film, and it is treatable. A sharp dark line along one edge, or dark patches visible inside the bead, means water has got behind or into the sealant. Only the first responds to cleaning.
2. For surface mould, clean the film off rather than just bleaching the stain.A bathroom cleaner or a degreaser first, worked with a soft brush along the joint, to remove the soap and oil. Bleaching without removing the film kills what is there and leaves the food behind, so it returns in weeks.
3. Then treat the stain, once, with a mould remover.A proprietary mould spray, or kitchen roll soaked in dilute bleach laid along the joint for an hour so it stays in contact. Rinse thoroughly afterwards. Use one product only — never combine bleach with anything acidic.
4. Accept that deep staining does not come out.Once the mould has grown into the surface of the silicone the pigment is in the material. It will lighten and not clear. At that point the choice is to live with it or replace the bead.
5. Fix the ventilation, because that is the actual control.Of the four things mould needs — food, moisture, warmth and time — moisture is the only one you can practically remove in a bathroom. An extractor that runs during and after showering, or a window open afterwards, does more than any product.
6. Dry the joint after use.A squeegee down the tiles and a cloth along the sealant line takes fifteen seconds and removes both the water and much of the film. It is the single most effective habit for making a bead last.
7. Clean the joint weekly as a matter of routine.It takes a minute with the same cleaner you use on the rest of the bathroom, and it stops the food source ever accumulating. A joint that never carries a film never grows mould on it.
8. Replace rather than clean where water is getting behind it.A lifted edge, a split, or black inside the bead means the seal has failed and the mould is in a place you cannot reach. Cutting it out and resealing is the only answer, and cleaning it repeatedly is wasted effort.

Tips

• A dark line that appears exactly along the tile edge, in a perfectly straight line, is nearly always water behind the bead rather than mould on it.
• Bathroom cleaners that leave a residue give the mould something to eat. Rinse after cleaning rather than leaving product on the joint.
• Silicone that has been repeatedly bleached goes chalky, loses elasticity and splits sooner. Aggressive cleaning shortens the life of the bead.
• Mould on the grout as well as the sealant points at a room-level humidity problem, not a sealant problem. Deal with the ventilation.
• A shower used by several people back to back never gets a chance to dry. Spreading showers out, or leaving the extractor running longer, makes a measurable difference.

Common mistakes

• Buying a more expensive sanitary silicone and expecting it to solve it — The fungicide protects the sealant, not the soap film on top of it. The new bead goes black on the same schedule unless the ventilation and cleaning change.
• Bleaching without cleaning first — It removes the stain and leaves the food source. The mould is back within weeks, and the bleach has degraded the silicone in the meantime.
• Mixing bleach with a limescale remover — Bleach plus acid releases chlorine gas within seconds in an enclosed bathroom. This is the most dangerous thing anyone routinely does in a bathroom.
• Cleaning a bead that has water behind it, repeatedly — The colony is inside the joint where nothing reaches. It has to be cut out.

Keeping on top of it

• Squeegee the walls and wipe the sealant line dry after showering (after each use)
• Clean the sealant with a bathroom cleaner and a soft brush (weekly)
• Run the extractor for the recommended overrun, and check it is actually extracting (every shower)
• Check the bead for lifted edges or splits (every few months)

Questions people ask