Black mould on silicone: clean it or replace it

Do the clean first, and do it once, properly. A mould and mildew product, left on for as long as its label says, with a mask, gloves and goggles on and the window open. Those are the government guidance's own precautions for removing mould. Then rinse, dry the bead with a cloth, and look at it in good light.

Three things can be in front of you.

• The bead is white and smooth again. The mould was growing on what sat on top of the silicone. It came off because it was never inside. Keep it that way with the next section.
• The surface is clean but the black is still there, under it. The cleaner has done what it can. The colour is in the silicone now, and no spray reaches it. This is the case HG means when it says its products cannot remove mould in the sealant.
• The edge has lifted, or there is black behind it. Run a fingernail along both edges of the bead. If it lifts away from the bath or the tile, water has been getting underneath, and the mould is growing where no cloth will ever touch it. Homebuilding & Renovating describes exactly this: edges lifting away, moisture trapped under them, and black mould growing there.

The last two are the same verdict. The bead comes out. Sealant makers are as blunt as the cleaner makers here: Unibond says to try a mould remover and, if that is not effective, replace the sealant altogether. A lifted edge is also a gap that lets water under the bead, so it is a seal that has stopped sealing, and resealing the bath is the job in front of you.

Never put bleach and vinegar on the same bead. UKHSA, the government's health security agency, states that bleach mixed with an acidic product produces chlorine gas, and names vinegar as one of those acids. Mixed with a cleaner containing ammonia, it gives off chloramines, which UKHSA describes as highly irritating to the respiratory tract. The pages ranking for this search print a bleach method and a vinegar method on the same page. In a small bathroom with the door shut, trying one and then the other is exactly how the two meet.

Removing it naturally. Vinegar and bicarb are what most people reach for first. Unibond, which makes sealant, says these and bleach "may work to some degree" but "do not kill the spores and have the potential to damage the sealant bead", and points you at a specialist mould and mildew remover instead. Be fair to the vinegar about the second half of that. Unibond's own data sheet lists good resistance to conventional cleaning products, so damage is a possibility its how-to raises, not a fact its data sheet records.

Bleach. The government's damp and mould guidance settles it in one line: "Mould and mildew products should be used in preference to bleach, for health and safety reasons." That is the same advice for mould on walls, and it holds on a bath edge too.

WD-40. People ask, and the maker does not claim it. The WD-40 Company's UK site lists what its Multi-Use Product is for, and mould, mildew and cleaning silicone sealant are not on the list. I would not put a lubricant on a bead you want to come up clean.

Whatever you use, mould that is in or under the silicone stays where it is. No recipe changes the verdict in the section above. It only changes how long it takes you to reach it.

Do not run new silicone over the old. Unibond's sanitary sealant data sheet says residues of old sealant "as well as mould on the substrate must be removed completely" before the new bead goes on, and the surface must be clean, dry and free of dust and grease. GE Sealants adds the reason it matters for mould in particular: leaving the old sealant in place "traps mildew pockets". A new bead over a mouldy one is the same mould with a fresh coat on top.

So the job runs in this order. Cut and strip every bit of the old bead, which is its own job and getting old silicone off without wrecking the bath covers it. Clean the joint with a mould and mildew product, rinse it, and let it dry right out. Then reseal it yourself, with the right tools to hand, or pay someone to do it. Fill the bath half full of water first, as Bostik's and Everbuild's data sheets both say; the full sequence is in how to reseal a bath. Use a sanitary silicone, and which sealant suits a bath or shower has the detail on what those labels mean.

Then let it cure before it gets wet. Unibond's own advice is to check the product's instructions for how long it needs before water contact, and how long silicone takes to dry before the shower goes back on has the detail.

And if there is half a cartridge left at the end, a screw-on cap over the spout keeps it from setting in the tube until you need it next.