Why does the sealant round my bath keep going black? How to re-seal it properly in a Birmingham home

You've scrubbed it, you've bleached it, you've gone at it with an old toothbrush — and the black line along the edge of the bath or shower tray is still there. That's because it isn't dirt sitting on top of the sealant; it's mould that has rooted into and behind the silicone itself, and no amount of cleaning will lift it once it's that far in. The only real fix is to strip the old sealant out and lay a fresh bead properly. This guide, from the Property Edge bathrooms team, explains why it keeps happening across Birmingham, Solihull and Wolverhampton homes, and how to re-seal it so it actually lasts.

Why bathroom sealant turns black in the first place

Silicone sealant fills the gap where the bath, basin or shower tray meets the wall — a join that flexes every time you fill the bath or stand in the tray. Mould grows there for three reasons that usually overlap: the room stays warm and damp, the silicone is constantly wet and slow to dry, and any tiny split in the bead lets water sit behind it where you can't see or clean it. Once mould takes hold inside that hidden gap, the black you see on the surface is just the tip of it. It's the same damp-air problem behind window and corner mould — our guide on stopping condensation and black mould covers the wider fix for the whole room.

Can you just seal over the top? (No — and here's why)

It's tempting to run a fresh bead of silicone straight over the old mouldy one. Don't. New silicone won't bond properly to old silicone, so it lifts within weeks — and worse, you've now sealed the existing mould in, where it carries on growing and bleeds back through. Every neat re-seal starts the same way: get all of the old sealant out first. That's the slow, fiddly part, but it's the part that decides whether the job lasts two years or two months.

How to re-seal a bath or shower properly — step by step

1. Strip out every scrap of the old sealant

Run a sharp blade or a plastic sealant-removal tool along both edges of the bead and peel it away in lengths. Pick out the stubborn bits with the blade — you want bare, clean surfaces with no silicone smears left, because new silicone won't stick to old residue. A silicone-remover gel (left on per the tin) softens anything baked-on. Take your time here; it's 80% of the job.

2. Kill the mould and let it dry completely

With the gap open, treat it with a fungicidal wash or mould remover and let it work, then wipe out. Now dry the whole area thoroughly — a hairdryer on the join helps. This is the step people skip, and it's the one that matters most: lay silicone onto a damp, mouldy gap and you've simply built it a new home. Bone-dry and mould-free is the target before a drop of new sealant goes on.

3. Mask, fill the bath, and lay one clean bead

Run low-tack masking tape along the wall and along the bath edge, leaving a consistent gap for the bead. Then — the trade trick most people miss — fill the bath with water before you seal it. A full bath sits lower; sealing it empty means the bead stretches and splits the first time you fill it. Use a good-quality mould-resistant (sanitary) silicone, cut the nozzle small, and run one steady bead in a single pass.

4. Tool it, pull the tape, and leave it alone

Smooth the bead with a wetted finger or a sealant tool in one firm stroke, then peel the masking tape away straight away while the silicone is still wet — that's what gives you the crisp line. Leave the water in the bath and don't use the bath or shower until the silicone has fully cured: check the tube, but it's usually 24 hours, not the 2–3 hours it takes to skin over. Rushing the cure is the second most common reason a re-seal fails.

How to keep it white for longer

• Run the extractor fan every shower and leave it going 15 minutes after — most bathroom mould is really a ventilation problem.
• Wipe the bath and shower edges dry after use, or squeegee the screen, so water isn't sitting on the silicone all day.
• Leave the bathroom door open after washing when you can, to let the moist air clear.
• Use mould-resistant (sanitary-grade) silicone, not cheap general-purpose — it's a couple of pounds more and resists regrowth far longer.
• If the mould keeps coming back across the whole room and not just the sealant, the room itself is too damp — that's a ventilation fix, not a silicone one.

When it's more than just the sealant

Sometimes the black line is a symptom of something bigger. If the silicone keeps failing in the same spot, water may already be getting behind the tiles or under the tray — and that can rot the wall, lift the tile adhesive, or show up as a damp patch on the ceiling below. If the tiles sound hollow, the grout is crumbling, or there's any movement in the bath or tray, re-sealing alone won't hold. That's worth our bathrooms team taking a proper look at, and if there's already damp tracking into the wall or ceiling our damp-proofing surveyors can tell you how far it's gone before it costs more to put right.

When to call us

Re-sealing a bath is a genuinely doable afternoon job, and if you fancy it, the steps above are exactly how we'd do it. But if the tiles or grout are going as well, if you've re-sealed before and it keeps failing, or you'd simply rather it was done once and done right, it's a quick, low-cost visit — sealant renewal is everyday work for our general repairs team, and a tired bathroom is bread-and-butter for our bathrooms team. Book a bathrooms visit across Birmingham, Solihull, Coventry, Wolverhampton and the wider Midlands, or get in touch and send a photo — we'll tell you honestly whether it's a re-seal or something more.