How to Reseal Bath Silicone That Has Gone Black

Black spots along the seal where the bath meets the tiles are not dirt. They are colonies of Cladosporium and Aspergillus that have grown their hyphae straight into the silicone rubber, feeding on the plasticisers and soap scum that soaked in over the years. This is why bleach spray fools you for a week and then the black is back. You are treating the surface. The fungus is inside.

The only proper fix is to cut it all out and put fresh silicone in. It takes an afternoon, costs roughly £15 in materials if you have nothing, and if you do it carefully once you will not have to think about that bath edge again for six or seven years. Rush it and the new bead lifts within months.

Why the black comes back no matter what you spray

Sanitary silicone contains fungicides (usually a combination of OIT and a triazole) at manufacture, but these leach out within roughly three to five years in a warm, wet bathroom. Once the biocide is gone, the porous surface of aged silicone is a perfectly nice substrate for mould, especially where soap film has accumulated. The organism grows into the polymer matrix itself. Bleach breaks down the pigment but not the living structure, so within days the new growth colours up again.

A useful reference here is the NHS guidance on damp and mould, which is worth reading if anyone in the house is asthmatic or immunosuppressed. Removing the seal in a small bathroom kicks up spores, so open the window and wear a cheap FFP2 mask while you cut.

What you actually need on the bench before you start

The single biggest reason DIY resealing goes wrong is people try to do it with just a knife and a tube. You need a proper kit or you will get a wavy bead and a wet substrate. My working list, tested over four bathrooms:

• Stanley knife with a fresh blade (a used blade drags and tears the old silicone rather than slicing it)
• Silicone remover gel such as Everbuild Silicone Eater or Unibond Silicone Remover, about £6 for a 100ml tube
• Plastic scraper, never metal against enamel or acrylic
• Isopropyl alcohol at 99%, one small bottle, roughly £4
• Blue low-tack masking tape, 25mm width (not the beige stuff, it lifts paint)
• Sanitary silicone, mould-resistant grade, one 310ml cartridge. Dow 785 or Everbuild Forever White are the two I keep going back to
• Skeleton gun, a decent one at £8 rather than the £2 one that jams
• Fugi smoothing tool (about £10) or a set of profiled rubber spatulas. Do not use a wet finger, you will end up with a lumpy edge and silicone on the tiles
• Kitchen roll and nitrile gloves

The dissolver gel is the item people skip. It is the one that saves you thirty minutes of scraping and, more importantly, gets residue out of the pores of the tile grout so the new bead actually bonds.

The removal, step by step

1. Fill the bath with water. Cold is fine. This sounds strange but it is essential: the extra weight pushes the bath down into its final loaded position, so the new seal is laid at the actual gap width it lives at. Skip this and you get a hairline crack the first time someone gets in.
2. Score both edges of the old bead with the Stanley, one clean run along the tile edge and one along the bath edge. Angle the blade almost flat to the surface. You are aiming to release the seal, not to gouge acrylic.
3. Peel out the loose ribbon of silicone. If it comes away in a long strip you did the score correctly. If it is fragmenting, your blade is blunt.
4. Apply the dissolver gel to whatever residue is left in the joint. Give it the full 30 to 60 minutes the tube specifies. Do not touch it in the meantime.
5. Scrape off the softened residue with the plastic scraper. Rag it out of the corners with kitchen roll wrapped round a lolly stick.
6. Wipe the joint down with isopropyl at least twice, using clean paper each time. This removes soap film, residual dissolver, and any oils from your fingers. The bond depends on this.
7. Let it dry fully. An hour with the extractor running is usually enough. Applying silicone to a damp substrate is the number two reason for premature failure after a dirty joint.

Masking, gunning, and getting a clean bead

Run one strip of blue tape along the tile roughly 3mm above the joint and another along the bath 3mm below it. The 3mm gap is the finished width of your bead. Use a small ruler if your eye is not calibrated. The tape is the difference between something that looks professional and something that looks like a five-year-old iced a cake.

Cut the cartridge nozzle at 45 degrees. Trim it just wide enough that a steady squeeze produces a bead that slightly overfills the 3mm gap. Practise on cardboard first. The gun should move at roughly one centimetre per second, no faster, and the trigger pressure needs to be constant. Do the whole run in one pass if you can. Restarts show.

Smooth with the Fugi tool, held at a consistent angle, in one continuous pull from one end to the other. Wipe the tool on kitchen roll every 30cm. Then, before the silicone starts to skin (you have about 5 to 8 minutes with most cartridges), lift the masking tape at a shallow angle away from the bead. This is oddly satisfying and gives you the crisp edge that separates a good job from an obvious repair.

Curing, first bath, and what to do if it goes wrong

Sanitary silicone skins in about 15 minutes and cures at roughly 2mm per 24 hours. A standard bead needs a full 24 hours before it sees water, and I would not fill the bath and get in for 48. Rushing this is the third common failure mode.

If, after a week, you see a hairline gap opening at the tile edge, you either did not fill the bath during application or the substrate was still slightly damp. There is no fixing this by adding more silicone on top, it will not bond. Cut the whole bead out and start again. Painful but true.

If the bead has skinned oddly, with a puckered or matt surface, it usually means the tube was near end-of-life. Sanitary silicone has a shelf life of around 12 months from manufacture and cartridges sat on a shed shelf for two years will not cure properly. Check the date on the crimp before you buy.

When resealing is not the right job

If the silicone is fine but the tiles behind the bath have loose grout, or you can see any daylight or dark shadow behind the panel, water is getting past the seal already and rotting the timber studwork underneath. Resealing over the top of that is a cosmetic patch on a plumbing problem. Pull the panel off and look before you commit to a cartridge. a family friend, who ran a domestic repair round for thirty years, used to say the difference between a repair and a bodge is usually whether you looked at the thing behind the thing first. That still applies.

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