DepositProofDIY

How to Remove Mouldy Bath Sealant and Apply Fresh Silicone

The DIY Man

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Old, discoloured, or mouldy silicone sealant around your bath or shower tray is one of the most common reasons UK letting agents deduct money from tenancy deposits. Leaving black mildew untreated can lead to water leaking into floorboards or neighboring flats, while a rough repair job looks unsightly on checkout day.

This guide walks you through removing damaged sealant safely without scratching acrylic tubs, treating underlying spores, and laying a crisp, watertight bead of fresh silicone using tools available from Screwfix, B&Q, or Wickes.

Difficulty Level: Intermediate

Deposit Risk: High (Black mould and peeling sealant are top targets for letting agent inventory checks)

Time Required: 1 hour work time (plus 24 hours cure time)

Estimated Cost: £10–£15

Tools & Materials Checklist

Tools

  • Standard Cartridge / Caulking Gun (approx. £3–£5)
  • Sealant Removal Knife or Plastic Scraper (avoids scratching bath surfaces)
  • Utility Knife / Stanley Knife
  • Sealant Finishing Tool / Profiling Tool (or a smooth wooden lolly stick)
  • Old Toothbrush & Cleaning Cloths

Materials

  • Sanitary Grade Anti-Mould Silicone Sealant (e.g., UniBond No More Moulds, Dow Corning 785, or Evo-Stik Sanitary) — Ensure it is White or Clear to match your existing fixture.
  • White Spirit or Methylated Spirits (for surface degreasing)
  • Low-Tack Masking Tape
  • Household Bleach or Mould Spray

Renter Tip: Never use cheap multi-purpose acrylic caulk in the bathroom. It is not waterproof and will wash away or turn mouldy within weeks. Always look for Sanitary Silicone with added fungicide.

Step-by-Step Instructions

Step 1: Drain & Dry Completely

Before touching any tools, ensure the bath or shower tray has been unused for at least 12 hours. Moisture trapped behind old sealant will prevent new silicone from bonding.

Step 2: Cut and Slice Out the Old Silicone

  1. Take your Stanley knife and run the blade flush along the top tile edge where it meets the silicone.
  2. Make a second flush cut along the edge touching the bath rim.
  3. Hook under one end with a plastic scraper and pull the old silicone strip away in continuous lengths.
  4. Use a plastic scraper (never metal on acrylic baths) to scrape away stubborn silicone residue until the surface feels completely smooth.

Step 3: Neutralise Mould Spores & Clean

Removing the rubber strip often reveals hidden black mould underneath.

  1. Scrub the joint using an old toothbrush dipped in pure bleach or mould spray.
  2. Let it sit for 15 minutes to kill deep-rooted spores, then wipe thoroughly with clean water.
  3. Pour a small amount of white spirit onto a clean rag and wipe the entire joint. This removes residual grease, soap scum, and silicone oils.
  4. Allow the area to dry completely for 30 minutes.

Step 4: Mask Your Edges (The Secret to Neat Lines)

If you haven’t used a caulking gun before, masking tape guarantees a professional edge:

  1. Apply a strip of low-tack masking tape along the tiles, roughly 3mm to 4mm above the joint.
  2. Apply a second strip along the bath or shower tray rim, 3mm to 4mm out from the corner.
  3. You should now have a neat 6–8mm exposed channel running around the bath edge.

Step 5: Prepare and Load the Cartridge

  1. Snip the plastic threaded nib off the top of the silicone tube using your utility knife.
  2. Screw on the clear plastic nozzle.
  3. Cut the tip of the nozzle at a 45-degree angle, ensuring the hole matches the width of your tape gap (around 5mm).
  4. Load the tube into your caulking gun and pull the trigger until silicone fills the nozzle tip.

Step 6: Apply the Silicone Bead

  1. Hold the caulking gun at a 45-degree angle to the wall.
  2. Starting in the furthest corner, press the trigger smoothly while moving the nozzle along the gap at a steady speed.
  3. Aim to push the silicone into the joint rather than just laying it on top. Apply one continuous line from corner to corner.

Step 7: Smooth the Sealant & Peel Tape

  1. Lightly spray the bead with a mix of water and a drop of washing-up liquid (this stops silicone sticking to your smoothing tool).
  2. Draw your sealant profiling tool (or wet finger/lolly stick) firmly along the bead in one continuous motion to create a neat concave joint and remove excess material.
  3. Peel off both strips of masking tape immediately while the silicone is still wet. Pull the tape away slowly at a 45-degree angle.
  4. If pulling the tape leaves small raised ridges, gently smooth over the bead one last time with a wet finger.

Crucial Curing Time

  • Do not get the sealant wet for at least 24 hours.
  • Keep bathroom windows open or extractor fans running to allow solvent fumes to disperse.
  • Check the cartridge instructions; some rapid-cure sealants allow shower use after 6 hours, but leaving it a full day ensures maximum adhesion for your move-out inspection.

Expected Inventory Result

A clean, white, mildew-free silicone bead with smooth edges passes standard checkout inventory checks under the “Clean & Well-Maintained” rating, completely eliminating £100+ contractor deduction threats.