Try cleaning it before assuming the sealant needs replacing — black mould on silicone is very often a surface growth that comes off completely, leaving sound sealant underneath. Bathroom and kitchen silicone provides exactly the warm, humid, organically-fed environment mould favours, and it is one of the most common places in a house for it to appear, even on sealant that is otherwise doing its job perfectly well.
Why silicone attracts mould specifically
Standard bathroom silicone sealant is porous enough at a microscopic level, and sits in exactly the conditions — consistent moisture, warmth, and often limited ventilation — that mould needs to establish itself. It is not a sign the silicone was applied badly or is a poor product; it is simply the most mould-prone material in most bathrooms, doing an otherwise perfectly good job of sealing a joint.
Cleaning it first
- Ventilate the area
Open a window or run an extractor fan — you will be using a cleaning product with fumes worth not breathing concentrated.
- Apply a mould-specific cleaner or diluted bleach
A dedicated mould and mildew spray, or a diluted bleach solution applied with a cloth or spray bottle, both work well on silicone specifically.
- Let it sit
Leave the product on the affected area for the time specified on the packaging — this is usually longer than people expect, often ten to fifteen minutes, and rushing this step is the main reason a first attempt looks like it has not worked.
- Scrub gently with an old toothbrush
The small bristles reach into the texture of the sealant bead far better than a cloth alone.
- Rinse thoroughly
Clean off all traces of the cleaning product afterward, particularly if you used bleach.
- Repeat if needed
Deeply set staining sometimes needs a second treatment. If two thorough attempts leave a visible stain even though the mould itself is gone, that is a cosmetic issue at that point, not an active mould problem.
When it needs replacing instead
| Sign | What it means |
|---|---|
| Mould keeps returning within days of cleaning | Growth has penetrated into the sealant itself, not just the surface |
| Sealant has pulled away from the tile or bath edge | The seal has failed and needs replacing regardless of mould |
| Sealant feels soft, sticky, or is crumbling | Material has deteriorated and lost its sealing properties |
| Persistent staining that will not lift with cleaning | Cosmetic only if the seal itself is otherwise sound — a judgement call |
| Water damage or softness in the wall behind the sealant | Points at a failed seal that has been letting water through — investigate promptly |
Removing and replacing a silicone bead
- Cut along both edges of the old bead
Use a utility knife to score along where the silicone meets the tile and the bath or basin on both sides.
- Peel out the old sealant
Pull it away by hand where you can, using the knife or a dedicated sealant removal tool for stubborn sections.
- Clean the joint thoroughly
Remove all residue, then clean the area with isopropyl alcohol or a silicone-specific surface cleaner and let it dry completely — new silicone will not adhere properly to a damp or residue-coated surface.
- Apply painter’s tape on both sides
This gives you a clean, straight edge and makes the finished bead look considerably more professional with very little extra effort.
- Apply new silicone in one continuous bead
Cut the nozzle at an angle sized to the gap, and run it in one smooth, continuous motion rather than stopping and starting, which leaves visible joins.
- Smooth it
Run a wetted finger or a purpose-made silicone smoothing tool along the bead in one motion to create a neat, slightly concave finish.
- Remove the tape immediately
Pull the tape away before the silicone skins over — waiting until it has started to cure risks tearing the fresh bead along with the tape.
- Leave it undisturbed to cure
Avoid running water over it or using the shower or bath for the time specified on the product, typically 24 hours, to let it cure fully before it faces moisture.
Keeping it from coming back
- Ventilate the bathroom well after showering — an extractor fan run for twenty minutes afterward makes a real difference.
- Wipe down the sealant occasionally as part of normal bathroom cleaning, rather than only addressing it once mould is visible.
- Consider a mould-resistant silicone specifically formulated for wet areas when replacing a bead — it costs little more and genuinely resists regrowth better than standard sealant.
- Squeegee the bath or shower screen after use if condensation and standing water are a persistent issue in the room.
