Can I put new silicone over the old layer?

Why it will not stick

Cured silicone has extremely low surface energy. Fresh silicone does not really bond to it. What you get is a cold joint: two layers with no chemical weld. Water climbs between them by capillary action. Mould grows where you cannot see it, behind the “new” bead.

A tube that claims it can go over old silicone is a bodge. In proper joint work it does not count as a seal.

When yes and when no

No if the bead is stained, torn, peeled or has mould in the mass. All of it out.

No if you see a pin-hole in a relatively new joint. Do not cap the hole. Cut a generous length and rebuild that segment on a clean substrate.

Yes try a deep clean if the joint is intact and only dirty. Replace when mould is in the mass or the flank opens.

The film you cannot see

Scraping the bead with a knife is half the job. A thin film of old silicone stays on the tile and the sanitaryware. Three months later the new bead peels like rubber: adhesion failure.

After the coarse cut: a silicone remover, dwell time, scrape without scoring the acrylic, degrease with a technical cleaner (not oily white spirit) and dry fully. One grain of old silicone is a bump and a weak spot.

On a face that was soaked in silicone, I sometimes open the meeting by a millimetre so the new bead hits a clean edge. A real 4–6 mm bond face takes movement; a smear over film does not.

How to renovate properly

1. Cut the old mass without wrecking the tray or the glaze.
2. Break the film with a silicone remover, not vinegar: vinegar does nothing to cured silicone.
3. Clear the residue, degrease, dry. If moisture weeps from the joint, wait. Sealant on wet is a leak waiting.
4. New backer rod if the old one was dirty or crushed.
5. Sanitary silicone with body — OTTOSEAL S100 here — tooled so water can run off.

Expensive mistakes

Leaving “a bit” in the corners. Capping a hole. Sealing over moisture from an opened joint. Thinking you save time: between two layers mould grows faster than on an old bead you can see.

Exceptions

No professional system promises a lasting seal on old silicone. A segment replacement is a segment replacement, not a patch on top. Wait for cure (often 24–48 hours on a typical shower joint, longer if it is deep) before you load it with water.