Silicone gone black in the bathroom isn't just unsightly — it's a sign the seal has failed. Water runs behind the joint, into the wall, into the floor. Renew the joint properly and you have 5 to 8 years of peace. Here's how we do it.

When does a joint really need renewing?

Three signs: (1) the silicone is black or grey in several spots — that's mould inside the material, no longer surface-cleanable. (2) it's lifting at an edge or cracked. (3) you can see moisture or discolouration in the wall plaster behind the joint. In all three cases cleaning won't help — the joint has to come out.

Step 1: Remove the old silicone completely

This is the step most DIYers do too quickly. Just scraping out "the black part" and laying new silicone over it gives you the same black streak in 6 months — the mould spores live deep in the substrate. The joint must come out completely, down to the last corner.

Tools: a sharp utility knife and a plastic silicone-removal tool (CHF 10 at any hardware store). First cut both sides of the joint with the knife, then pull out the strip. Scrape any residue with the removal tool until the tape edge or tile is completely bare.

Step 2: Clean and degrease

Now comes the step that decides success or failure: cleaning. Use a commercial silicone remover (chemical, not vinegar) on a lint-free cloth. Wipe both flanks of the joint twice — first pass dissolves the residue, second pass removes the grease. Then wipe with a dry cloth and wait at least 30 minutes until everything is dry.

Important: if the surface is wet, the new silicone won't bond. For bathroom renovations it pays to clean the night before and lay the silicone the next day.

Step 3: Mask cleanly

A perfect silicone line doesn't come from the gun — it comes from the tape. Apply quality painter's tape (Tesa Premium or equivalent — not standard masking, which lifts) on both sides of the joint at exactly the same distance from the gap. Recommended joint width: 4 to 6 mm.

Tip: don't just press the tape down — run your fingernail along the inner edge. Otherwise silicone runs under the tape and you end up with a wavy line.

Step 4: Apply the silicone

Use sanitary silicone with fungicide — not generic silicone from the discounter. The fungicide is the active ingredient that keeps mould away for years. Standard colours: white, Manhattan grey, anthracite, Bahama beige. For coloured joints, look at the old joint first or take a tile sample to the hardware store.

Cut the cartridge tip at an angle — the angle should roughly match the joint width. Pull the gun evenly through the joint without stopping, from one corner to the next. If you have to interrupt, plan the break in a corner where there's a visual seam anyway.

Step 5: Smooth and pull the tape immediately

As soon as the silicone bead is laid, smooth it immediately — either with a smoothing tool or with a wet finger (water with a drop of dish soap). One single continuous movement. Then, while the silicone is still wet, peel the tape away from the joint at an angle. If you wait until it sets, you'll tear the clean edge.

Common mistakes

  • Universal silicone instead of sanitary: moulds in 4 months.
  • Pulling the tape after smoothing fails: the edge tears and looks ragged.
  • Too little silicone: looks fine from the front but has voids behind — water still runs through.
  • Splashing water on freshly applied silicone: disrupts the cure.

When professional help pays off

For a single joint in the bathroom, doing it yourself is worth it. For a whole bathroom (shower, bath, basin) you're looking at 3-4 hours quickly — and a beginner usually needs twice that for a perfect result. We do a complete standard bathroom in 2 hours for around CHF 320, with quality sanitary silicone, a clean line, and 5-8 years of peace on the result.