Mould on sealant and grout: the quick answer
If the mould on your sealant wipes away after treatment, keep treating it and keep the area dry. If dark spots stay put, the mould has grown into the silicone itself and no spray will reach it. Cut the sealant out and replace it: a tube of sanitary silicone costs about £5.
That is not the answer most product pages give you, but it is the honest one. Silicone is soft and slightly porous. Once mould gets into it, you are cleaning the surface of something that is stained from the inside. Grout is a little different, and we cover it below.
Treat or replace your sealant: how to decide
Treat it when
- The mould is a surface film or fresh spots
- It wipes away after a 10 minute treatment
- The sealant is firmly stuck, with no gaps
- The bead is not split, cracked or peeling
Replace it when
- Dark spots do not shift after treatment
- The sealant is peeling, split or coming away
- Water is getting behind it
- It has been treated many times and keeps going black
The “dark spots that do not shift” test is the important one. A lighter stain is not the same as a clean one: bleach can fade the colour while the mould is still there.
And even the good stuff has limits. Mould resistant silicone helps, but it is not a magic bullet. It will still go mouldy after a while, especially in a bathroom that stays damp.
How to remove mould from silicone sealant
If the sealant is sound and the mould is on the surface, this is all it takes:
- Open the window or put the extractor fan on, and put on gloves and eye protection.
- Spray the sealant with a mould remover spray and leave it for 10 minutes.
- Wipe it clean with a damp cloth. Mould Check needs no rinsing.
- Dry the joint with a towel and let the room air.
- Check it the next day. If dark spots remain, the mould is inside the silicone: replace it.
Avoid scrubbing silicone hard with a stiff brush or a scourer. It roughens the surface, which gives mould more to hold on to next time.
How to replace mouldy sealant (about £5)
If the mould is inside the silicone, the fix is simple: cut the sealant out and run a new bead. You need a tube of white sanitary silicone and a skeleton gun, about £5 together, plus a sharp blade or sealant remover tool, masking tape, and a smoothing tool if you have one.
- 01
Cut out the old bead completely
Use a sharp blade or a sealant remover tool to slice along both edges and pull the old silicone out. Get every bit: new silicone will not stick properly to old, and leftover mould shows through. - 02
Clean and treat the joint
Spray the empty joint with mould remover, leave it 10 minutes, then wipe it out. This deals with mould that has grown behind the old bead. - 03
Let it dry completely
Ideally leave it overnight with the window open or fan running. Silicone over a damp joint will not bond and traps moisture. - 04
Mask with tape
Run masking tape along the tiles and the bath or tray, leaving a neat gap for the bead. This is the secret to a straight line. - 05
Run a new bead
Cut the nozzle at an angle, then run one steady, continuous bead along the joint. Tip: fill the bath with water first so it sits at its heaviest, then empty it once the sealant has cured. - 06
Smooth it, but not with a licked finger
Use a smoothing tool or a gloved finger dipped in soapy water. Do not lick your finger. You are pressing bacteria straight into the sealant you just bought to keep mould out. - 07
Remove the tape
Pull the tape away slowly while the silicone is still wet, angled away from the bead. - 08
Let it cure before you shower
Check the tube for the cure time, often 24 hours. Keep the shower and bath dry until then.
Mould on grout: clean it, regrout or seal it
Treating mould on grout
Grout is a hard surface, so mould on grout usually responds better than mould on silicone. Spray the grout lines, leave them 10 minutes, then wipe. While the grout is still wet, a soft brush along the lines is fine to lift stubborn marks. Rinse the brush, dry the tiles and let the room air.
When to regrout
If the grout is crumbling, cracked, missing in places or stays dark after several treatments, it is time to rake out the affected lines and regrout. Damaged grout lets water get behind the tiles, which is a bigger problem than how it looks.
Sealing grout
Once grout is clean and fully dry, a grout sealer makes it less absorbent, so it takes on less water and stains less. It is worth doing on new grout and after a deep clean. Like anti-mould silicone, it slows things down rather than making mould impossible.
How to stop mould coming back on sealant and grout
Every product on this page, ours included, is fighting the same thing: water sitting on surfaces for hours. Mould comes back unless the moisture is dealt with. These habits make the biggest difference:
Squeegee or wipe after showers
Thirty seconds takes the standing water off tiles, grout and sealant.
Extractor fan on, and left running
Keep it going after your shower, not just during it.
Leave the door open to dry
Let damp air out into the rest of the home once you are done.
Wash or replace the shower curtain
A mouldy curtain keeps reseeding the corners of the tray.
If mould is spreading beyond the shower onto walls and ceilings, read how to get rid of mould on walls, and if it keeps coming back through paint, see our guide to painted over mould.
Which mould spray is best for sealant and grout?
Honestly, on tiles and grout a bleach-based spray like HG mould remover works too. If you are happy with bleach, use it on its own, ventilate well, and never mix it with anything else. Our guide on why you should never mix bleach and vinegar explains why.
Where Mould Check earns its place is everything around the tiles. It is hypochlorous acid at pH 6 to 7, not bleach, so it is gentler on silicone sealant, safe on painted bathroom walls and ceilings, and gives off no harsh fumes. That matters in a small bathroom with no window, or a home with children, pets or a pregnant tenant. You spray it, leave it 10 minutes and wipe it clean, with no rinsing. It is COSHH free, though we still recommend gloves and eye protection.
What a spray cannot do
No spray, ours included, gets mould out from inside silicone. If the spots stay after treatment, the £5 tube of sealant is the right answer. For a full comparison of options, see our black mould remover guide.Ready to treat it? Shop the Mould Check mould remover spray, available as a 500ml spray and 5L and 10L refills.
Questions people ask about mould on sealant and grout
Mould Check is not bleach. It is hypochlorous acid at pH 6 to 7, so it does not strip colour the way bleach does, and it is safe on sealant and grout. As with any cleaning product, keep it off clothes, wipe up drips, and test a hidden patch first if your grout is coloured.
With bleach-based sprays, no: open a window or run the extractor fan, because bleach gives off fumes in a small bathroom. Mould Check gives off no harsh fumes, so it is safe to use with people at home. Ventilating afterwards still helps, because it lets the sealant and grout dry, and dry surfaces are what keep mould away.
It depends on the surface. On hard tiles and grout, a bleach-based spray like HG mould remover works well if you ventilate. On silicone sealant, painted walls and in homes where people are in, a pH neutral, bleach-free spray like Mould Check is the better fit, because it is gentle on sealant and paint and gives off no harsh fumes. If the mould is inside the silicone, no spray will fix it: replace the sealant.
It slows it down, it does not stop it. Mould resistant sanitary silicone still goes mouldy in a bathroom that stays wet, especially if it was smoothed with a licked finger or applied over old mould. Squeegee after showers and keep the extractor fan running.
Check the tube, because it varies by product. Many sanitary silicones need around 24 hours before they get wet. Do not shower or run a bath until the time on the tube has passed.
No. New silicone does not bond properly to old silicone, and it seals any mould underneath in, so it shows through again. Cut the old bead out completely, clean and treat the joint, let it dry, then apply the new bead.