Black joints around the shower and bath are one of the commonest bathroom complaints. This guide explains why mould clings to silicone, what can be cleaned, when the joint has to be cut out and redone, and what three manufacturer data sheets say about application and curing. For mould on a wall or ceiling, see mould on walls.
Updated 4 November 2026. General information, not a substitute for a technician’s assessment.
The essentials
- Mould needs moisture (EPA). A joint that is wet every day with little air is the ideal place.
- Soudal says mould appears even on fungicide silicone when soap residue stays on the joint, together with moisture, poor airflow and warmth. “Anti-mould” does not remove the need to clean.
- Surface mould may come off with detergent and water or a dedicated product, followed by drying. Silicone stained right through usually needs cutting out and a new joint.
- Cured silicone can only be removed mechanically (Bostik) and is not paintable (Bostik). Do not seal over mould (EPA).
- Without ventilation the mould returns: bathroom extractor fan and ventilation.
Why mould grows on silicone
The silicone itself is not what feeds mould, what sits on it is: soap, shower gel, limescale. Soudal (FAQ) gives these causes: high humidity and poor airflow, organic residue, warmth, and silicone without anti-mould properties. The EPA notes that showers are hard to keep mould-free, and that running a fan or opening a window during the shower, and cleaning more often, usually keeps it down.
What the fungicides promise
| Data sheet | Fungicide |
|---|---|
| Penosil Neutral Sanitary Silicone 350c (undated) | “Added fungicide gives excellent resistance to mould”; result 0+ in the ISO 846 test (we do not interpret the scale) |
| Bostik SX Premium Sanitary Silicone (footer dated 22 October 2026) | “Anti-fungal compounds to prevent mould growth” |
| Soudal Trade Sanitary Silicone (revision 24 June 2022) | The sheet does not mention a fungicide |
No sheet gives a protection period in years. Soudal says its Stay Fresh range carries a guarantee against mould discolouration, but still needs regular cleaning.
Cleaning light mould
This suits silicone that is still white underneath.
- Ventilate and protect yourself. Open the window, wear rubber gloves and goggles (CDC).
- Start with detergent and water, scrub and dry completely (EPA). It also removes the soap the mould lives on.
- If a stain remains, use a dedicated product. Robbialac Removedor de Bolor e Fungos (909-0070, information dated 16 December 2021): spray, leave for 5 minutes, scrub, rinse with plenty of water and dry well. It has pH above 12.5, smells of chlorine, calls for gloves and may discolour paint: test first.
- Bleach, with care. On hard surfaces the CDC allows at most one cup of bleach in 3.8 litres (one US gallon) of water. The EPA says bleach is not recommended routinely and that dead mould can still cause allergies, so it must be removed.
- Never mix products. CDC: bleach with ammonia or other cleaners gives off a poisonous gas.
- Dry. The EPA advises drying whatever got wet within 24 to 48 hours.
Recipes with bicarbonate of soda and hydrogen peroxide, or vinegar, are everywhere on lifestyle sites. We found no test or data sheet that backs them.
When to cut out and redo
- The stain comes back within days even after cleaning.
- The silicone is cracked or loose. The Swiss FOPH says sealants lose elasticity, shrink and let water in, and should be renewed in time.
- The mould is behind the silicone, on the wall or the shower tray edge.
If mould has reached the wall, deal with the cause first (see mould on walls). The EPA says not to seal over mouldy surfaces: clean and dry first.
Steps to redo the joint
The figures come from the data sheets; the practical order is ours.
- Cut out the old silicone and pull it away. Remnants come off mechanically (Bostik); Penosil mentions a silicone remover if needed.
- Clean and let it dry. Penosil asks for dry surfaces free of dust and grease, and solvent with a clean cloth on non-porous ones.
- Application temperature: Penosil 5 to 40 °C, Soudal 5 to 35 °C, Bostik 5 to 40 °C.
- Joint size: Soudal gives 5 to 30 mm wide and 5 mm minimum depth. In deep joints Bostik asks to cut the depth to half the width with backer rod.
- On a bath, Bostik says fill it halfway before sealing and empty it only once the silicone has cured.
- Smooth with a tool wetted in soapy water before a skin forms (Bostik: within 5 minutes). Wet only the tool (Soudal).
- Let it cure.
How long curing takes
| Data sheet | Skin | Cure |
|---|---|---|
| Penosil 350c | 5 to 8 min | 2.0 to 2.5 mm per 24 h |
| Soudal Trade Sanitary | about 7 min | about 2 mm per 24 h |
| Bostik SX Premium | 5 to 6 min | about 3 mm per 24 h |
Values at 23 °C and 50 % relative humidity. This is the thickness that cures per day, from the surface inwards. Our own arithmetic: a joint 5 mm deep takes about 2 to 3 days. No sheet says when the shower may be used again, so the prudent course is to wait that long. Soudal’s silicone smells of vinegar (acetic) and is not for marble or granite.
Keeping mould away
- Ventilate during and after the shower. The EPA points to indoor humidity below 60 %, ideally 30 % to 50 %. See also musty smell in the home.
- Wipe joints and glass with a squeegee or cloth.
- Clean joints with a non-abrasive product (Soudal) so soap does not build up.
- Do not paint over it: anti-mould paint does not replace cleaning and ventilation. For tiles, see painting tiles and ceramic.
When to call a professional
If the stain returns after redoing the joint and ventilating, if the shower tray edge feels soft, or if a stain appears on the ceiling of the room below (possibly a leak). The CDC points to higher risk for people with asthma, allergies or weak immunity. The EPA says that above about 10 square feet (about 1 m²) cleaning is, in most cases, no longer a do-it-yourself job.
What we could not confirm
- How long the silicone’s fungicide lasts. No sheet gives a period.
- That harsh cleaning strips the fungicide. We saw the claim from a supplier, not on a data sheet.
- When to use the shower after a new joint: the period above is our own calculation.
- The best home recipe. No test found.
- Cement tile grout: we only have the EPA’s guidance for hard surfaces.
- Guidance from the DGS or SNS 24 on mould in joints and bleach mixtures: not found.
- Prices of silicone and labour.
Sources (read on 4 November 2026)
- U.S. EPA, Mold Cleanup in Your Home: detergent and water, no sealing over mould, about 10 square feet limit.
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home: ventilating the shower, indoor humidity, drying in 24 to 48 hours.
- CDC (USA), Mold: About: bleach, not mixing with ammonia, protection, people at risk.
- Soudal, FAQ: I have used anti mould silicone, why is there mould appearing.
- Soudal, Trade Sanitary Silicone, technical data sheet, revision 24 June 2022.
- Penosil, Neutral Sanitary Silicone 350c, technical data sheet, undated.
- Bostik, SX Premium Sanitary Silicone, technical data sheet, footer dated 22 October 2026.
- Robbialac, Removedor de Bolor e Fungos 909-0070, technical data sheet, 16 December 2021.
- Swiss Federal Office of Public Health (FOPH), Bolor em espaços habitáveis (mould in living spaces), 2021, cited through our guide mould in wardrobes and cupboards, not re-read today.