Why the sealant goes black when the tiles stay clean
Glazed tile is a hard, non-porous, sealed surface. Mould can sit on it, but it cannot get into it, which is why a wipe with almost anything takes it off and it stays off for a while.
Silicone sealant and cement grout are different materials with a different problem. Both are slightly porous, both stay wet longer than the tile around them because water collects in the joint, and both accumulate a film of soap residue, skin cells and shampoo — which is food. Mould that lands there does not just sit on the surface. It puts out hyphae, fine root-like threads, that grow down into the material.
That is the whole reason sealant is harder to deal with than tile. By the time you can see a black speckle, the visible part is the fruiting body and there is a network underneath it that your cloth never reaches.
Cleaning and bleaching are not the same job
This distinction matters more here than anywhere else in the house.
Household bleach is very good at removing the colour from mould. Wipe neat bleach on black sealant and within ten minutes it looks dramatically better. What has happened is that the pigment in the visible growth has been oxidised. The hyphae below the surface are largely untouched, because bleach is mostly water and the chlorine reacts at the surface rather than travelling into the silicone.
Two or three weeks later the black is back, in the same spots, and people conclude the mould is unusually stubborn. It is not — it was never removed, only decolourised. This is the single most common reason people believe bathroom mould is impossible to beat.
A product designed for the job needs to stay in contact with the joint long enough to work into the material rather than run off it, which is why gels and clinging formulations tend to outperform thin sprays on vertical sealant lines.
Treating it properly
Dry the joint first
Applying anything to a wet joint dilutes it immediately. Leave the bathroom unused for a few hours, or run the extractor fan, and wipe the sealant dry with kitchen roll before you start. This one step makes a bigger difference than the product choice.
Apply, and then leave it alone
Dwell time is the active ingredient. Whatever you use, the instructions will specify a contact time, and cutting it short is why treatments underperform. Apply along the joint, close the door, put the extractor on, and go and do something else for the stated period rather than scrubbing after two minutes.
Our Eradicator Mould Remover Spray is made for exactly this kind of hard-surface work on sealant, grout and tiles. Follow the contact time on the label rather than rinsing early.
Work the grout separately
Grout has an advantage sealant does not: it will take gentle abrasion. After the dwell period, a stiff nylon brush — an old toothbrush is fine for one joint, a grout brush for a whole wall — lifts residue out of the surface texture. Do not do this to silicone. Abrasion scratches it, and every scratch is a new place for growth to establish.
Rinse and dry
Rinse the whole area with clean water and dry it. Leaving treatment residue on the joint does not provide ongoing protection and can attract dirt.
When cleaning is the wrong answer
Sometimes the sealant is beyond treatment, and continuing to clean it is wasted effort. Replace rather than clean if:
- The black is within the silicone rather than on it — visible through the surface, unchanged after a proper treatment with full dwell time
- The bead has lifted, split, or pulled away from the tile or the bath at any point along its length
- You can push the sealant and feel it move independently of the substrate
- There is staining on the wall or ceiling below the joint, which suggests water is already tracking behind it
That last one is the important one. A failed seal around a bath or shower tray lets water into the wall structure, and no amount of surface cleaning addresses that. The mould you can see on the sealant is then a symptom, not the problem.
Replacing it
Cut the old bead out completely with a sealant knife — completely, not mostly, because new silicone will not bond to old silicone and any residue left behind carries the existing growth into the new joint. Clean the channel, let it dry thoroughly, and use a sanitary-grade silicone containing a fungicide.
Fill the bath with water before you seal around it, and leave it full until the silicone has cured. The weight spreads the joint to the width it will actually sit at in use; seal an empty bath and the bead tears the first time someone gets in.
If you are not confident cutting out and re-running a long bead neatly, this is a reasonable job to pay a tiler or bathroom fitter for. A poorly formed joint fails quickly and lets water in, which costs considerably more than the labour.
Keeping it clear afterwards
Mould returns to sealant because the conditions return, not because the treatment failed.
- Run the extractor fan during the shower and for fifteen to twenty minutes afterwards. Approved Document F sets a minimum intermittent extract rate of 15 litres per second for bathrooms in work covered by the Building Regulations; an old or blocked fan often falls well short
- Squeegee or wipe the shower enclosure and the sealant line after use — thirty seconds, and it removes the water the growth depends on
- Keep the bathroom door shut while showering and for a while after, so the moisture leaves through the fan rather than migrating into cooler bedrooms
- Deal with soap residue regularly, since that is the food source
A word on safety
Bathrooms are small, enclosed and often poorly ventilated, which makes them the worst room in the house to use strong chemicals carelessly. Ventilate while you work, wear gloves, and avoid breathing the spray directly.
Above all, never mix bleach with any other cleaning product, particularly anything containing ammonia or acid such as a limescale remover. The combination releases toxic gases and does so quickly in a confined space. If you have already treated a joint with one product, rinse thoroughly and let it dry before applying anything else.
