Black speckling along the bath or shower silicone is one of the most common things we get asked about, and one of the most misunderstood. Once mould has taken hold inside a silicone bead, cleaning it is cosmetic. The growth lives in the sealant and in whatever is damp behind it, so the black returns as soon as the room steams up again. Putting it right properly is a two hour job with the right materials, but only if you also deal with the reason the joint stays wet.
Why the black keeps coming back
Mould needs moisture, a food source and time. Silicone that has been cleaned with abrasive products has a scratched, porous surface that holds soap residue perfectly. Once spores are rooted below the surface, bleach only bleaches the visible layer. Two or three showers later the colour returns and people conclude the sealant is cheap, when what actually happened is that nothing was removed.
The four real causes
- Sealant applied over sealant. New silicone does not bond to cured silicone, so it lifts at the edges and water gets behind it. Every reseal that skips full removal fails.
- Wrong product. General purpose or frame sealant has no fungicide. In a bathroom it will colonise within a year.
- Movement. A bath sealed empty, or a shower tray on a flexing floor, tears the joint open a fraction every time it is used. Water sits in that crack permanently.
- The room never dries. Weak extraction means surfaces stay damp for hours after every shower, which is all mould needs.
How to reseal a bath or tray properly
- Cut the old bead out completely with a plastic sealant remover, working along both faces. Do not leave a smear.
- Scrape off residue, then wipe the whole joint with methylated spirit or a silicone remover solvent and let it dry fully. Any grease or soap film ruins the bond.
- Check what is behind. If the wall board is soft or the tile edge is loose, stop. Sealant will not fix wet substrate.
- Fill the bath with water, or stand in the shower tray, so the unit sits at its working weight.
- Mask both sides with low tack tape, apply one continuous bead of sanitary grade silicone, tool it in one pass with a wetted profiling tool, then pull the tape straight away.
- Leave it 24 hours before use, and leave the bath full until the surface has skinned.
Where silicone is the wrong answer
People often try to seal a gap that is far too wide, usually where a bath sits away from a bowed wall. A bead over about 8mm will slump, crack and go mouldy no matter how carefully it is applied. In that situation the fix is a bath seal trim, a tile upstand or packing the bath out to the wall first. Similarly, silicone squeezed into a cracked grout line is patching over a tanking failure, and it will keep failing until the shower area is made watertight behind the tiles.
Stopping it coming back for good
- Run the extractor throughout the shower and let it overrun for at least fifteen minutes afterwards.
- Squeegee the tiles and glass and wipe the sealant line dry. Twenty seconds a day is more effective than any spray.
- Keep the bathroom door shut while showering so the moisture goes out of the fan rather than into the landing.
- Clean with a mild detergent rather than abrasive creams that score the surface.
- If the joint is still failing yearly, get the ventilation and any movement under the bath or tray looked at rather than resealing again.
When it is a symptom of something bigger
Persistent mould at the wall junction of a tiled shower usually means water is already behind the tiles. Loose or hollow sounding tiles, dark grout that never dries and a springy feel underfoot all point the same way. At that stage the honest answer is to strip the shower area, replace any blown board, tank it properly and re-tile. We would rather tell you that at survey than take money for a reseal we know will not hold.
Get a straight answer on yours
Send a photo of the joint through our enquiry form and we will tell you whether it is a reseal or something behind the tiles. We carry out sealing, retiling and full refits across Stoke-on-Trent with our own insured fitters, and you can read more on the repairs page.
Common questions
Why does bleach only work for a week?
Bleach removes the colour of the mould but not the growth rooted inside the silicone. The black comes back as soon as the surface stays damp for a day or two.
Can I put new silicone on top of the old?
No. New sealant will not bond to cured silicone, so it peels within months and traps the old mould underneath. The old bead has to come out completely.
Should the bath be full when I seal it?
Yes. Fill it before sealing and leave it full until the silicone has skinned. That forms the joint at maximum deflection so it does not tear the first time someone gets in.
Is sanitary silicone actually different?
It contains a fungicide that resists colonisation for far longer than general purpose sealant. It is a few pounds more per tube and worth every penny in a bathroom.
How often should sealant be replaced?
Treat it as a consumable and expect five to seven years. If yours is failing every year, the cause is ventilation, movement or substrate, not the sealant brand.