How to Remove Old Silicone Before Applying New Waterproofing

Removing old silicone before applying new waterproofing is one of those jobs where preparation makes all the difference. If you leave any residue behind, your new sealant or membrane won't bond properly — and in a wet area, that means leaks, mould, and potentially costly damage. The good news is that with the right tools and a bit of patience, most homeowners can tackle this themselves.

Why Proper Silicone Removal Matters

Silicone is non-porous and doesn't bond to itself reliably, so applying new silicone or a waterproofing membrane over old silicone is a recipe for failure. In NZ bathrooms and wet areas, the NZ Building Code requires wet areas to be properly waterproofed (refer to Clause E3 — Internal Moisture). A failed seal can allow water to penetrate wall linings, subfloors, and framing — damage that can be expensive and, in some cases, require building consent to repair.

Tools and Materials You'll Need

• Sharp utility knife or silicone removal tool (a dedicated plastic scraper helps avoid scratching tiles)
• Silicone remover solution (chemical softener)
• Stiff nylon brush or old toothbrush
• Methylated spirits or isopropyl alcohol
• Clean cloths or paper towels
• Masking tape (for neat edges when re-sealing)
• Safety glasses and gloves

You'll find most of these at Bunnings or Mitre 10. Look for dedicated silicone remover products in the sealants aisle.

Step-by-Step: How to Remove Old Silicone

Step 1 — Score and Cut the Silicone

Using a sharp utility knife, carefully score along both edges of the silicone bead where it meets the tile, bath, or substrate. Keep the blade as flat as possible to avoid gouging the surface. Work slowly — rushing this step is the most common cause of scratched tiles or damaged grout.

Step 2 — Pull Out the Bulk of the Silicone

Once scored, grip the end of the silicone bead with your fingers or needle-nose pliers and peel it away in one long strip if possible. In older bathrooms, the silicone may be brittle and come away in pieces — that's fine, just be thorough.

Step 3 — Apply a Chemical Silicone Remover

For stubborn residue, apply a silicone remover solution (such as those made by Sika or similar brands) directly to the remaining silicone. Follow the manufacturer's dwell time — usually 30 minutes to a few hours. The chemical softens the silicone, making it much easier to scrape away without damaging the substrate.

Step 4 — Scrape Away the Residue

Use a plastic scraper or nylon brush to work away the softened residue. For tiled surfaces, avoid metal scrapers where possible. For fibreglass or acrylic baths and shower trays, always use plastic tools — metal will scratch.

Step 5 — Clean the Surface Thoroughly

This step is critical. Wipe down the entire joint area with methylated spirits or isopropyl alcohol on a clean cloth. This removes any remaining silicone oils, soap scum, or grease that would prevent the new waterproofing from bonding. Allow the surface to dry completely — in a NZ bathroom, this may take longer in winter or in poorly ventilated spaces.

Step 6 — Inspect the Substrate

Before applying any new waterproofing, check the substrate carefully. Look for:

• Mould or mildew in the joint or behind the old silicone (treat with a suitable mould killer and allow to dry)
• Cracked or crumbling grout (re-grout before sealing)
• Damaged wall linings — if you find soft or damaged GIB plasterboard or substrate, this will need to be repaired or replaced before waterproofing

Choosing the Right New Waterproofing Product

Once the surface is clean and dry, your choice of new product depends on the application:

For full wet area waterproofing membranes, products from Ardex and Mapei are well regarded in the NZ trade. Always follow the manufacturer's instructions for primer requirements and cure times.

Safety Notes

• Work in a well-ventilated space — silicone removers and methylated spirits produce fumes
• Wear safety glasses and chemical-resistant gloves
• Keep products away from children and pets
• Check WorkSafe NZ guidance if you're unsure about handling chemical products safely

When to Call a Professional

While silicone removal is generally a DIY-friendly job, there are situations where you should bring in a qualified waterproofer or tiler:

• Extensive mould or water damage behind tiles or linings — this may indicate a larger leak requiring investigation
• Full wet area waterproofing in a new build or renovation — depending on the scope, this may require a building consent and must meet NZ Building Code Clause E3. Always check with your local council
• Damaged substrate or wall linings that need replacing — work involving structural elements may require a Licensed Building Practitioner (LBP)
• If you're unsure whether your waterproofing is up to standard, BRANZ provides excellent technical guidance for NZ conditions

A qualified local waterproofer or tiler can be found on tradedirectory.co.nz — it's always worth getting a professional opinion if you have any doubts about the extent of the damage.

Sources & Further Reading

• MBIE Building Performance — NZ Building Code
• BRANZ — Weathertightness and Moisture
• Sika NZ — Sealants and Waterproofing
• Ardex NZ — Waterproofing Systems
• WorkSafe NZ — Safe Use of Chemicals
• Licensed Building Practitioners