Black speckles creeping along your shower sealant or bathroom grout are almost always mould. Wiping it makes it fade, then it returns in days. This guide explains why that happens, what genuinely removes it versus what only masks it, and the ventilation habits that stop it regrowing. Many Regents Park flats are period conversions with limited airflow, which makes this a recurring battle worth winning properly.
Why mould keeps coming back
Mould is a living organism, not just a stain. It grows where three things meet: moisture, warmth, and something to feed on such as soap residue. Bathrooms provide all three. The reason it returns after cleaning is that most people remove the visible surface but leave the roots embedded in porous grout or flexible sealant. Kill the surface only, and the colony regrows from within.
What actually removes it
- On sealant and grout: a mould-and-mildew remover, or household bleach diluted in water, kills the organism. Contact time is everything; it must sit, not be wiped straight off.
- The method: apply, leave for the time the product states, then rinse and dry. For stubborn patches, lay bleach-soaked kitchen roll along the sealant line so the solution stays in contact.
- When sealant is stained through: if the black is inside the silicone, no cleaner reaches it. Cut out the old sealant and re-apply fresh; this is the only real fix at that stage.
Kills it versus hides it
| Approach | Result |
| Bleach/mould spray with dwell time | Kills surface growth |
| Quick wipe with a cloth | Hides it briefly, regrows fast |
| Painting over it | Masks it, then bleeds through |
| Replacing sealant | Removes embedded colonies |
Safety first
Never mix bleach with other cleaners, especially anything containing ammonia or acid such as limescale removers; the combination releases dangerous gases. Ventilate the room, wear gloves, and avoid breathing the spray. Do the mould treatment before any acid descaling, and rinse fully between the two.
A real example: the recurring shower line
A tenant bleaches the shower sealant every fortnight, yet the black returns. The problem is not the cleaning; it is the moisture that never leaves. The bathroom has a window that stays shut and no extractor running long enough. Once they start leaving the window open after showers and wiping the sealant dry, the two-week cycle stretches to months, and eventually stops. Cleaning treated the symptom; ventilation treated the cause.
Stopping it returning
Mould cannot grow on a dry surface. The whole prevention strategy is about drying the room quickly after use.
- Run the extractor fan during the shower and for 15 to 20 minutes after.
- Open a window to clear steam where there is no fan.
- Squeegee or towel the walls, screen, and sealant after showering.
- Keep the shower door or curtain open between uses so air circulates.
- Fix dripping taps and seals; constant damp feeds mould.
Common mistakes and how to fix them
- Wiping instead of dwelling. The cleaner needs minutes on the surface. Fix: apply and walk away before rinsing.
- Treating stained-through sealant with spray. It will never clear. Fix: re-seal instead.
- Cleaning but not ventilating. Guarantees regrowth. Fix: dry the room every time.
- Mixing chemicals. Risks toxic fumes. Fix: use one product at a time and rinse between.
Action checklist
- Ventilate the room before you start.
- Apply mould remover to sealant and grout; leave for the stated time.
- Rinse and dry the surfaces.
- Assess sealant; if black is embedded, plan to cut out and re-apply.
- Run the fan or open a window after every shower going forward.
- Squeegee walls and dry the sealant daily.
Conclusion and next step
Bathroom mould is not a cleaning problem so much as a moisture problem with a cleaning symptom. Kill the current growth with dwell time, replace sealant that is stained through, then win permanently by drying the room after every use. Your next step: after your next shower, open the window or run the fan and wipe the sealant dry. That one habit does more than any spray.
FAQ
Is bathroom mould bad for my health?
The NHS notes that damp and mould can affect breathing and may worsen problems for people with asthma or allergies. It is sensible to remove it and keep the room dry, especially in a small bathroom.
Why does mould keep coming back after I bleach it?
Usually because the room stays damp, or the mould is rooted inside porous grout or silicone. Bleach clears the surface but not the moisture source or embedded colony.
Can I paint over mould to cover it?
No. Paint over live mould bleeds through and peels. You must kill and dry the area first, and replace stained sealant, before any decorating.
Do I need to replace the sealant or can I clean it?
If the black wipes to grey and fades, cleaning is enough. If it stays dark after treatment, the mould is inside the silicone and the strip should be cut out and re-applied.
References
- NHS — guidance on the health effects of damp and mould in the home.
