It’s tempting. The old bead looks a bit grubby, you have a tube of new silicone, and running a fresh line over the top takes five minutes. Some people who are paid to reseal do it too.
The problem is that it doesn’t work for long. New silicone won’t stick properly to cured silicone, so the top layer never truly bonds. Water and mould get between the layers, and the joint ends up worse than before. This guide explains why, what we find when we cut over-sealed joints out, and how old sealant should be removed before a new bead goes on.
The short answer
You can’t successfully put new silicone over old silicone. The new bead may look neat at first, but it is only sitting on the old one, not bonded to it. As the bath or tray moves and the joint gets wet, the edges lift, and water gets underneath.
The only reliable fix is to remove all of the old sealant, clean and dry the joint, and lay a single new bead onto bare, clean surfaces.
Why new silicone won’t bond to old
Cured silicone has a smooth, non-stick surface. That’s part of what makes it good at shedding water, but it also means very little will stick to it, including more silicone. Sealant needs to bond to clean, dry surfaces with no grease on them: the bath, the tile, the tray or the frame. Old sealant is none of those.
On top of that, an old bead is usually dirty, carrying soap film and often mould. Even if the new layer gripped a little, it would be gripping onto grime.
What happens underneath
When we cut out over-sealed joints, we usually find the same things:
- mould growing between the two layers, showing as dark shadows through the new bead;
- water trapped in the gap, sometimes running behind the bath or tray;
- the original sealant split or lifted, which is why it was covered in the first place;
- a thick, uneven joint that is hard to clean and looks messy.
In a 1950s terrace in Three Bridges, Crawley, we were called to a bath where someone had sealed over failed sealant. Mould was growing underneath. We removed both layers, cleaned and treated the joint and resealed with Everbuild anti-fungal silicone. The whole job took under three hours.
Over-sealing vs full removal
| Sealing over the top | Removing and resealing | |
|---|---|---|
| Bond | Sits on the old bead; doesn’t bond | Bonds to clean bath, tile or frame |
| Mould | Trapped underneath and keeps showing through | Removed and the joint treated |
| Water | Can run between layers | Sealed at the surface |
| Look | Thick, lumpy bead | One neat, even bead |
| How long it lasts | Often weeks to months | Depends on use and cleaning; far longer when done properly |
| Next reseal | Harder: two layers to dig out | Straightforward |
How old sealant should be removed
- Protect the surface. Baths, trays and worktops scratch easily. Use a plastic scraper where you can.
- Cut along both edges. Use a sharp blade or a sealant removal tool to cut the bead where it meets the bath and the tile, keeping the blade flat to the surface.
- Pull out the bulk. Most of the bead should come away in strips.
- Remove the film. A thin layer is always left behind. A sealant remover product and a plastic scraper take it off.
- Clean and degrease. Wipe the joint with a suitable solvent cleaner so no soap or residue is left.
- Treat mould. Use a mould remover on any staining in the joint, then rinse.
- Dry it fully. Leave it until it’s completely dry. Sealant won’t bond to damp surfaces.
Our guide on how to remove old silicone goes through this in more detail.
Need it done properly? Mr Mastic covers the South East, London and Scotland, with a one-year guarantee on every job. Call or email for a quick quote.
Mistakes to avoid
- Scraping with a metal blade held at an angle, which scratches acrylic baths and trays.
- Leaving a thin film of old silicone and sealing onto it.
- Resealing straight after cleaning while the joint is still damp.
- Using decorators’ caulk instead of silicone in a wet joint.
- Sealing a bath empty, so the bead tears when the bath is filled.
- Laying a thick bead to hide an uneven joint.
Signs your sealant has been over-sealed
- The bead looks unusually thick or lumpy.
- You can see an older bead, or a different colour, under the edge.
- Dark mould shows through the sealant but doesn’t clean off.
- The top layer peels away easily at the ends.
- A shower leaks even though it was “resealed” recently.
If any of these sound familiar, the joint needs stripping back. That’s what our remedial sealant service is for.
What about other sealants over silicone?
The same principle applies to most products. Decorators’ caulk, hybrid sealants and paint don’t bond well to silicone either. If a joint has had silicone in it, the silicone needs to come out, and the surfaces need cleaning, before anything else goes in. Silicone residue on tiles can also stop grout and paint from taking, which is another reason to remove the film fully.
Kitchens, basins and windows too
Over-sealing isn’t only a bathroom problem. We see the same thing along kitchen worktops, around basins and on window frames:
- Kitchens. A new bead run over shrunken worktop sealant leaves a channel underneath where water from the sink sits and soaks into the worktop edge. Shrunken sealant needs taking out instead. In a Maidenbower kitchen on Harper Drive, sealant around 30 years old had shrunk and let water behind the worktop; it was cut out and replaced with colour-matched sanitary silicone, with a food-safe product beside the sink.
- Basins. Layers build up where the basin meets the wall, and mould shows through the top coat.
- Windows. New mastic over cracked old mastic peels off in the first winter and lets rain behind it.
The fix is the same everywhere: take it all out, clean, dry and reseal once.
When to call a professional
If the old sealant comes away cleanly and there’s no mould underneath, removing and resealing a single bath is a manageable DIY job. Call a professional if:
- there are several layers to remove;
- the surfaces are delicate, such as acrylic, natural stone or coated glass;
- mould has spread behind the bead or into the wall;
- the joint keeps failing and you don’t know why.
We take all of the old sealant out, find out why it failed and reseal it properly. Every job has our one-year guarantee. Get in touch for a quick quote.
Quick answers
Can you put new silicone over old silicone?
No. New silicone doesn’t bond to old silicone, so it lifts and traps water and mould underneath.
Do I need to remove all of the old sealant?
Yes, including the thin film left on the tiles and bath. New sealant needs clean, dry surfaces to bond to.
How do I know if my bath has been over-sealed?
Look for a thick or lumpy bead, an older bead showing at the edge, or mould showing through that won’t clean off.
Can I put caulk over silicone?
No. Caulk, paint and most other sealants won’t bond to silicone either.
Can you fix a joint someone else over-sealed?
Yes. We remove both layers, clean and treat the joint, and reseal it with anti-fungal silicone.
Related services
More advice
- How to Remove Old Silicone Sealant
- Why Does Bathroom Sealant Go Mouldy?
- How to Reseal a Bath (and When to Call a Pro)
- Leaking Shower? How to Tell if It’s the Sealant
- All advice
This guide is general information about sealant work. It isn’t legal, fire-safety or structural advice; for those, speak to a qualified professional.