Silicone Sealant: Types, Technique, and What to Buy

Black mould creeping along the silicone around your bath isn't just ugly. It means water is getting behind the sealant, wicking into the timber frame beneath, and the longer you leave it the more damage it does. Replacing silicone costs under £10 in materials and an hour of your time. Replacing a water-damaged floor joist costs thousands. The difference between the two outcomes is choosing the right silicone and applying it properly, which is exactly what most guides skip.

What it is and what it's for

Silicone sealant is a flexible, waterproof compound that comes in a 300ml (sometimes 280ml or 310ml) cartridge. You load the cartridge into a sealant gun, cut the nozzle at an angle, and squeeze out a bead along any junction where two surfaces meet and water could get through. Think bath-to-tile, shower tray-to-wall, sink-to-worktop, window frame-to-brickwork.

It stays flexible once cured. That's the point. Grout is rigid and cracks when surfaces move (and they do move, especially baths on timber floors). Silicone absorbs that movement. It's why every tiler uses silicone at internal corners and change-of-plane joints, never grout.

Silicone is not paintable. If you need to seal a gap that will be painted over (skirting boards, architrave, window frames on the inside), use decorators' caulk instead. Caulk is paintable but not waterproof. Silicone is waterproof but not paintable. Pick the wrong one and you'll either have paint peeling off within weeks or water getting in where it shouldn't.

Types: acetoxy vs neutral cure

This is where most beginners go wrong. They grab whatever's cheapest on the shelf without reading the label. There are two fundamentally different cure chemistries, and using the wrong one can ruin the finish or corrode the surface you're sealing.

Acetoxy (acid cure)

The moment you squeeze acetoxy silicone out of the cartridge, you'll know it. It smells like vinegar. That acetic acid is a by-product of the curing process. Acetoxy silicone cures faster (tack-free in around 15-30 minutes), costs less, and works brilliantly on ceramic tiles, porcelain, and glass. Most bathroom sealant jobs use acetoxy.

But it has limits. The acetic acid corrodes bare metals, bonds poorly to cement-based surfaces, and will permanently stain natural stone. Marble, limestone, granite worktops: acetoxy silicone will etch the surface and leave marks you can't remove.

Neutral cure

Neutral cure silicone releases alcohol or oxime instead of acid. Little to no smell. It bonds well to acrylic baths, uPVC window frames, metals, and natural stone without causing damage. The trade-off is a slightly longer cure time and a higher price.

If you're sealing around an acrylic bath (most modern baths are acrylic), neutral cure is the safer choice. Acetoxy can bond initially but pull away from acrylic over time as the substrate flexes. Some products bridge this gap: the Dowsil 785N is a neutral cure sanitary silicone, giving you mould resistance without the acid.

Sanitary grade (anti-mould)

Any silicone labelled "sanitary" or "anti-mould" contains fungicidal additives that inhibit mould growth. This is what you want in bathrooms, shower rooms, and around kitchen sinks. Standard silicone without these additives will grow black mould within months in a humid environment, no matter how well ventilated the room is.

Anti-mould doesn't mean maintenance-free. Even the best sanitary silicone will eventually grow mould if the bathroom has poor ventilation and sealant surfaces aren't wiped down occasionally. The fungicides inhibit growth; they don't make the surface immune. Keep your extractor fan running for at least 15 minutes after showering.

LMN (Low Modulus Neutral)

LMN silicone is a specialist product for perimeter sealing around window and door frames on the exterior. "Low modulus" means it stretches more easily (typically ±25% of joint width) without pulling away, which matters because frames expand and contract with temperature changes. Standard silicone can't handle that much movement without debonding.

Dow 796 is the most widely available LMN product in the UK at around £11 per cartridge from Screwfix. Your window fitter will (or should) use this when fitting new frames. If they reach for general-purpose silicone, question it.

Choosing the right type

When in doubt, neutral cure sanitary silicone is the safe default. It costs a couple of pounds more per cartridge but works on every substrate without risk.

How to apply silicone properly

This is the section that separates a clean, professional-looking finish from the smeared mess that most first-timers produce. Application quality determines longevity as much as product choice. A rough, rippled bead traps water and soap residue, accelerating mould growth even on anti-mould products.

Removing old silicone first

Never apply new silicone over old. Water gets trapped between the layers, stagnates, and mould grows faster than it would on bare surfaces. Strip it completely.

1. Score along both edges of the old sealant with a utility knife or a dedicated silicone removal tool (a plastic scraper shaped like a spatula, around £3 – £5 from any DIY store).
2. Peel away the bulk. Most of it should come off in strips if you get under the edge.
3. Apply silicone remover gel (Everbuild, UniBond, or own-brand versions are all fine, around £5 – £7). Leave it for 15 minutes. It dissolves the residue.
4. Scrape off the softened remains. Use a plastic scraper on acrylic baths and GRP shower trays to avoid scratching.
5. Wipe down with methylated spirits on a cloth. This degreases the surface and removes the last traces. White spirit doesn't work for this.

The surface must be bone dry and free of any residue before you apply new sealant. Any moisture or contamination and the new silicone won't bond.

The fill-the-bath trick

This is the single biggest mistake beginners make with bath sealing, and it's barely mentioned outside trade forums. Acrylic and steel baths on timber-joist floors are particularly prone to it. Cast iron baths on solid ground floors are less affected, but filling first does no harm.

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Applying the bead

You'll need a skeleton sealant gun (the most basic type, £3 – £8 from any DIY store). Cut the nozzle at 45 degrees, about 8-10mm from the tip for a standard bath-to-tile joint. Too wide and the bead will be thick and wasteful. Too narrow and you won't get enough material down.

Mask both sides of the joint with painter's tape first. Run two strips of tape parallel to the joint, leaving the gap exposed. This gives you crisp edges and means you don't need to be surgically precise with the gun.

Hold the gun at roughly 45 degrees to the surface and apply even pressure. Push the bead ahead of the nozzle rather than dragging it behind. Move at a consistent speed. One smooth pass. Don't stop and start.

Smoothing

Dip your finger in a cup of water with a drop of washing-up liquid in it. Run your finger along the bead in one smooth motion, pressing the silicone into the joint and creating a concave profile. The soapy water stops the silicone sticking to your skin.

Pull the masking tape off immediately while the silicone is still wet. If you wait until it starts to skin over, the tape will pull the edge of the bead away and leave a ragged line.

Fugi kits (plastic profiling tools, around £5 for a set) are an alternative to the finger method and give a more consistent profile, especially on longer runs.

Cure time

Touch-dry doesn't mean water-safe. Most silicones feel dry to the touch within 15-30 minutes, but the cure hasn't completed. Wait a minimum of 24 hours before running water over freshly applied silicone. For a new bath or shower installation, 48 hours is safer. If your builder says it's fine to use the shower the same evening, it isn't.

How much do you need

A single 300ml cartridge of silicone covers approximately 12-15 linear metres of a standard 5mm-wide bead. In practice, that's roughly:

• One standard bath (3 sides) plus the basin: 1 cartridge
• A walk-in shower enclosure (3 walls): 1 cartridge
• Kitchen worktop-to-wall junction (typical L-shape): 1 cartridge
• Full bathroom (bath, shower, basin, toilet base): 2 cartridges

Buy one spare. Cartridges don't last long once opened. The nozzle clogs within a few days even with a cap on, and after a couple of weeks the silicone in the cartridge starts to cure from the top down. If you're doing a bathroom, buy two cartridges of the same colour and use them in the same session.

Colours and matching

White and clear are the two standard colours, and they account for 90% of domestic sealant use. White against white tiles and sanitaryware. Clear where the surfaces either side are different colours or where you want the sealant to disappear.

Beyond that, most manufacturers offer black, grey, and ivory. For tile-matching, BAL and Mapei sell "grout-effect" silicone that colour-matches their grout ranges, so the silicone at internal corners blends with the grout lines on the tile faces. If you're using a coloured grout, ask your tile supplier whether they stock a matching silicone.

For specialist colours (brown, beige, specific RAL numbers), companies like County Construction Chemicals stock one of the widest colour ranges in the UK and offer custom colour matching.

Cost and where to buy

Silicone sealant is stocked everywhere. Screwfix, Toolstation, Wickes, B&Q, Travis Perkins, Jewson. Prices vary more by brand and grade than by retailer.

The trade consensus is clear: Dowsil (formerly Dow Corning) 785 is the benchmark. Multiple tradespeople report 2-3 years of zero mould growth. Everbuild Forever White, which uses Microban antimicrobial technology, is the budget alternative with independently tested longevity claims. No Nonsense from Screwfix is acceptable for budget DIY but won't last as long as the Dow products in a heavily used bathroom.

A sealant gun is a separate purchase. Budget skeleton guns start at £3 from Toolstation. For a one-off bathroom re-seal, that's all you need. If you're doing multiple rooms or a kitchen installation with lots of worktop joints, a rotating-barrel gun like the WorkPro Professional (around £12 – £15) gives smoother control.

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Silicone vs decorators' caulk

This causes confusion because both come in a cartridge and both fill gaps. The rule is simple.

Silicone is waterproof but not paintable. Use it in wet areas: bathrooms, kitchens, around sinks and baths, exterior window frames.

Decorators' caulk is paintable but not waterproof. Use it in dry areas: where skirting meets the wall, around architrave, filling cracks before decorating.

If you use caulk around a bath, water will get through within weeks. If you use silicone around a door frame, you can't paint over it and any emulsion you try will bead up and peel off.

Do not seal the inside of shower screen frames

This one surprises people. Shower screen frames (the aluminium channel at the bottom of a glass panel) are designed to drain back into the shower tray. Water gets inside the frame, runs along the channel, and drips back into the tray through weep holes.

If you seal the inside edge of the frame to the tray with silicone, you block the drainage path. Water accumulates inside the frame with nowhere to go and eventually leaks out the back, running under the tray and onto the floor below. Seal the outside of the frame to the tiles or wall, but leave the inside edge free to drain.

Common mistakes

Applying over old sealant. The most common one. Peel it all off, clean with methylated spirits, start fresh. No shortcuts.

Sealing an empty bath. The bath drops when filled. The sealant cracks. Fill the bath with water, seal while full, wait 24 hours, drain.

Using general-purpose silicone in wet areas. It'll go mouldy in months. Always sanitary grade in bathrooms and kitchens.

Using acetoxy on acrylic baths. It bonds initially but can peel away over time. Use neutral cure for acrylic.

Using acetoxy on natural stone. The acid etches and permanently stains marble, limestone, and some granites. Use neutral cure only.

Cutting the nozzle too wide. You end up with a fat bead that's hard to smooth and wastes material. Start narrow. You can always cut more off.

Not masking. Tape takes two minutes. The clean edges it gives you are worth ten times that in time saved trying to clean up a messy bead.

Smoothing with a dry finger. The silicone sticks to your skin, pulls away from the joint, and the result is a mess. Use soapy water on your finger, or a profiling tool.

Where you'll need this

• Tiling - at internal corners and where tiles meet other surfaces
• Kitchen Installation - worktop-to-wall junctions, around sinks
• Second Fix Plumbing - around basins, toilets, and waste pipe entries
• Windows and Doors - perimeter sealing with LMN silicone
• Bifold and Sliding Doors - frame-to-structure perimeter sealing

Silicone sealant appears at multiple stages of any extension or renovation project, from structural weatherproofing around new window frames through to the finishing touches in a bathroom. Buy the right type for each substrate, strip the old stuff completely before re-sealing, and fill the bath before you seal around it. Those three things prevent 90% of the problems homeowners encounter with sealant.