Preventing and removing mould in silicone joints
Black or reddish spots in the silicone joint are almost always mould, caused by standing moisture and soap residue. If the joint is intact, an anti-mould spray applied directly removes the infestation. If it is porous, cracked or discoloured throughout, only complete renewal with a fungicide-treated sanitary silicone such as OTTOSEAL S100 helps.
Contents
What you need
Project template “Renew the silicone joint in the bathroom”Black or reddish spots in the silicone joint are almost always mould, caused by standing moisture and soap residue. If the joint itself is still intact, remove the infestation with an anti-mould spray that is sprayed on directly and works after just a few minutes. If the joint is porous, cracked or discoloured all the way through, however, the only remedy is to renew it completely with a sanitary silicone with fungicidal additives such as OTTOSEAL S100.
Why does mould form in the silicone joint?
Joints in the sanitary area are permanent hosts to moisture – ideal conditions for mould spores, which are in the room air everywhere anyway. Four causes occur most often in practice:
Standing moisture
After showering, water stays in the joint and dries off poorly – the breeding ground for mould is created.
Soap and shampoo residue
Organic residue on the joint additionally serves moulds as a source of food.
Inadequate ventilation
Without an exchange of air after showering or bathing, the humidity in the bathroom stays high permanently.
Silicone without fungicidal additives
Simple construction or window silicone has no mould-inhibiting additives and offers mould considerably less resistance than special sanitary silicone.
Solution 1: joint is intact – remove mould from the surface
If the silicone is still smooth, elastic and free of cracks, a targeted treatment with anti-mould spray is as a rule enough. The spray removes mould, fungus, algae and moss reliably and disinfects the joint at the same time.
Spray directly onto the infestation from a distance of approx. 5–10 cm – the mould does not have to be removed or roughened beforehand.
It starts working after just a few minutes. With heavy infestation, repeat the spraying after 10 minutes.
Then wash the joint and the adjoining tiles with plenty of clean water so that no residue remains.
If dark discolouration is still visible after drying, the mould has usually already grown into the silicone – then the only remedy is to renew the joint (see below).
Caution: The bleaching effect can discolour delicate wallpaper, carpets, textiles and wood – test beforehand at an inconspicuous spot. Store unopened packs cool (+5 °C to +25 °C) and dry.
Solution 2: joint is damaged – renew it completely
If the silicone joint is cracked, porous or black all the way through, the mould is usually already in the material itself. Cleaning then brings no lasting improvement – the joint has to be run again.
| Step | What to do | Check |
|---|---|---|
| 1. Remove the old silicone | Remove the joint mechanically and completely with a cutter or joint scraper, including remains on the flanks. | No silicone residue visible or detectable by touch |
| 2. Clean the substrate | Make the contact faces free of grease, clean and dry with Otto Cleaner F surface cleaner – the bonding faces have to be absolutely clean, otherwise mould settles on the flank again. | Substrate feels free of grease and grippy |
| 3. Reseal | Seal again with OTTOSEAL S100 – an acetoxy-curing 1K silicone sealant with fungicidal and bacteriostatic additives, in accordance with DIN 18545 part 2 group E. Application temperature +5 °C to +35 °C. | Even, bubble-free bead of sealant |
| 4. Tool the joint | Tool with smoothing agent and joint smoother before a skin forms – e.g. with the OTTO joint smoother spatula. | Smooth, closed surface with nothing for dirt to attack |
| 5. Let it cure | Let the silicone cure right through undisturbed before the joint gets wet. You can work out the material needed in advance with our Ottoseal silicone usage table. | Joint feels completely firm and dry |
You will find detailed instructions with colour choice and fields of application in the Guide: Which silicone for the shower?
Preventing mould permanently
Use fungicidal sanitary silicone
In the wet area use exclusively sanitary silicone with fungicidal additives, not construction or window silicones.
Dry off after showering
Remove water from tiles and joint with a squeegee and then ventilate well.
Mild cleaning instead of scouring agents
Aggressive cleaners can attack the fungicidal additives – mild cleaners protect the joint in the long term.
React early
Treat the first grey haze immediately with anti-mould spray instead of waiting until the coating takes hold.
Common mistakes – and how to avoid them
Only wiping it away on the surface
Anyone who only wipes mould off with a damp cloth removes merely the spores on the surface – the cause in the joint material remains.
Using the wrong silicone
Standard silicone without fungicidal additives offers mould considerably less resistance than special sanitary silicone such as OTTOSEAL S100.
Sealing onto a damp substrate
The bonding faces have to be absolutely dry and free of grease before sealing, otherwise the new silicone does not adhere permanently and moisture gets in again.
Spray untested on delicate materials
Anti-mould spray can discolour wallpaper, textiles and wood – always test first at an inconspicuous spot.
Frequently asked questions
How do I tell whether it is mould or just dirt?
Mould usually shows as black, grey or reddish spots or streaks that cannot be removed completely with a damp cloth. Plain dirt from limescale or soap, by contrast, can usually be wiped off without any problem.
Can I simply scrub the mould away?
Wiping away only the surface does not remove the spores permanently. A special anti-mould spray works into the joint and gets rid of the infestation more reliably than cleaning alone.
How long does the anti-mould spray have to take effect?
It starts working after just a few minutes. With heavy infestation the spraying should be repeated after about 10 minutes, before rinsing off with plenty of water.
When do I have to renew the silicone joint completely?
As soon as the joint is cracked or porous, or the mould stays visible even after treatment with anti-mould spray, it is usually in the material – then the only remedy is complete removal and resealing.
How do I stop the mould coming back?
Use sanitary silicone with fungicidal additives, squeegee off the water after showering, ventilate well and use mild rather than aggressive cleaners so that the fungicidal additives are not attacked.
Is fungicidal silicone a health concern?
According to TÜV Rheinland (TOXPROOF), OTTOSEAL S100 is considered a particularly recommendable low-pollutant building product and is suitable for applications in accordance with IVD data sheet no. 3.
You will also find suitable products directly in our category sanitary silicone.
More guides on this topic
All guides on “Problems with silicone joints”
Instructions
Guide: cleaning silicone joints – what helps and what harms
Clean silicone joints properly: scratch test, household remedy check and deep clean
Problem solving
Guide: silicone joint is pulling away from the wall – causes and fix
Why silicone joints come away from tile, bath or frame and how the new joint holds
Problem solving
Guide: silicone joint is turning yellow – causes and fix
Yellow silicone joint: clean off the film or renew the joint, depending on where the discolouration sits