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The short answer
Black spots on the silicone sealant round a bath, shower tray or basin are mould growing in the warm, humid air of the bathroom. If the mould sits on the surface, treat it with a bathroom mould and mildew remover, following the label. If the stain is inside the bead and will not shift, the sealant has to be cut out and replaced. Either way, the mould returns unless you dry the room out after every bath or shower.
What you are looking at is usually this:
Black or grey spots along the sealant bead at the bath, shower tray or basin
Spots that come back weeks after cleaning
Staining that sits under the surface of the silicone and does not wipe off
Why it happens
The US EPA's guide to mould and moisture puts it simply: the key to mould control is moisture control. Mould grows where materials stay wet; the EPA says that if damp areas are dried within 24 to 48 hours, in most cases mould will not grow.
A sealant bead is in the wettest spot of the room. UniBond, which makes bathroom sealants, notes on its cleaning guide that silicone is applied where sinks, baths and shower trays meet a tiled wall, to stop water seeping behind them, and that the warm, humid bathroom creates ideal conditions for mould on it.
UK government guidance on damp and mould in the home names condensation as one of the most common causes of damp and mould: moisture from everyday activities such as showering, drying laundry and cooking builds up in the air when it cannot be removed. The same guidance warns that simply removing surface mould will not prevent it from reappearing.
How to fix it
- Protect yourself. The EPA advises gloves and goggles without ventilation holes when cleaning mould, and an N-95 respirator. Open a window or run the extractor fan while you work. UniBond also asks for protective gloves and a well-ventilated room when you use solvents to remove sealant.
- Try a mould remover on the bead. UniBond says the best solution is a specialist bathroom mould and mildew remover, formulated to kill mould spores. Follow the product label for contact time and rinsing. Use one product at a time and do not combine cleaners: the EPA warns never to mix chlorine bleach with cleaners that contain ammonia, because toxic fumes could be produced.
- Decide whether to replace. UniBond says replacement is needed where the mould has set too deeply into the sealant. If dark staining is still visible inside the bead after cleaning, cleaning again will not reach it.
- Remove the old sealant. UniBond's removal guide describes applying a silicone sealant remover, leaving it for 3 hours, then gently scraping the softened bead away with a strong plastic tool. For stubborn residue it suggests a cloth soaked in white spirit or methylated spirit left for a few hours, then a plastic tool or scouring pad.
- Clean and dry the joint. UniBond says removing all the residue helps prevent mould and is essential, because new sealant will not adhere unless the surface is clean and dry. Wipe the joint with methylated spirit before resealing and let it dry fully. The EPA is explicit: do not paint or caulk mouldy surfaces; clean up the mould and dry the surface first.
- Apply new sealant. Run a steady bead along the joint, smooth it with a sealant tool and wipe away any excess straight away, as UniBond describes.
- Let it cure before using the bath or shower. Curing times vary by product, so check the cartridge. UniBond gives one of its sealants as touch dry in 20 minutes and fully dry within 24 hours.
Stop it coming back
New sealant goes mouldy again in a room that stays wet. The EPA recommends running the bathroom fan or opening the window when showering, cleaning bathrooms frequently, and keeping indoor humidity below 60 percent, ideally between 30 and 50 percent. UK government guidance lists the same habits for occupants: use extractor fans in kitchens and bathrooms, keep internal doors closed while showering or cooking, open windows, cover pans and dry laundry outdoors when you can. Its checklist for landlords also asks whether bathroom extractor fans are fully working and powerful enough to remove the moisture from showering and bathing.
Check that it worked
After the new bead has cured, look at it every week or two. Clean white silicone that stays clean means the moisture is under control. Spots returning within weeks point to a ventilation problem rather than a sealant problem: check that the extractor fan actually runs and pulls air, and that the room dries out after use.
When it is more than the sealant
US and UK guidance draw the line at different places. The EPA says that if the mouldy area is less than about 10 square feet (roughly a 3 ft by 3 ft patch), in most cases you can handle the job yourself; beyond that, or after a lot of water damage, it points to its remediation guidance for larger jobs and experienced contractors. UK government guidance says larger areas of mould should only be dealt with by qualified professionals.
If mould spreads from the sealant onto walls or ceilings, or if you rent and the extractor fan does not work, tell your landlord: the UK guidance is written for rented housing providers. It notes that damp and mould mainly affect the airways and lungs, and that people with respiratory conditions, weakened immune systems, children, older people and pregnant women are more at risk.
Mould on sealant is a small, visible sign of a damp room. Clean what can be cleaned, replace what cannot, and the fix will only last if the bathroom dries out after every use.
Questions about this page
Will bleach get mould off silicone sealant?
UniBond, a sealant maker, advises against vinegar, baking soda and bleach on mouldy sealant: it says they do not kill the spores and can damage the bead. It recommends a specialist bathroom mould and mildew remover. Never mix chlorine bleach with cleaners that contain ammonia, which the US EPA warns can produce toxic fumes.
Can I run new sealant over the mouldy bead?
No. The US EPA says not to paint or caulk mouldy surfaces: clean up the mould and dry the surface first. UniBond also says new sealant will not stick unless the surface is clean and dry, so the old bead and its residue have to come off.
Why does the mould keep coming back?
Mould grows where moisture stays. UK government guidance says removing surface mould alone will not stop it reappearing; the moisture from showering and bathing has to be removed, for example with a working extractor fan.