Selleys Ask an Expert: Sealants for Every Home Repair

Choosing a sealant: the four questions that decide it

Almost every sealant question that reaches Selleys can be resolved by answering four things about the joint. Work through them in order and the product usually picks itself.

1. What are the two surfaces either side of the joint?

This is the question that most often gets skipped, and it is the one that most often causes a failure. Substrate compatibility is specific and it is published. No Mould, an acetic cure silicone, is not suitable for metals other than aluminium and stainless steel, because the acetic acid released as it cures can corrode other metals. Almost every Selleys sealant excludes polyethylene, polypropylene and Teflon. Around a mirror or on sensitive substrates, a neutral cure product is the lower-risk choice.

2. Does it need to be painted?

If yes, a silicone is out. Every Selleys silicone data sheet carries the same limitation: not paintable — complete painting seven days prior to applying sealant. The hybrid and SMP-based products are the paintable ones, and even they have conditions. 655FC and 3 in 1 take water-based paints only; for a solvent-based paint you must apply a water-based paint first as a primer.

3. How much will the joint move?

Movement capability is a published number and it varies enormously across the range:

Brick & Concrete Silicone	±50% (ISO 9047 Method B)
No Mould	±25% (ISO 9047)
No More Gaps Exterior	±25% (ISO 9046)
3 in 1	±20% (ISO 9046 modified)
650FC	±20%
No More Gaps Interior Multipurpose	Up to 15% (ISO 9046)

4. Will it be immersed in water?

This is where the most consequential limitations sit, and they are absolute. Not for permanent water immersion appears on No Mould, Wet Area Silicone, 655FC and others. Not for below waterline applications appears on Brick & Concrete Silicone. A sealant that is fine around the top of a pool is not fine below the water line, and the data sheets say so plainly.

Acetic cure or neutral cure?

Silicones cure by one of two chemistries and they are not interchangeable.

Acetic cure silicones — No Mould among them — release acetic acid as they cure, which is the vinegar smell for the first day. They bond well to glazed ceramic tile, glass and porcelain, and they are the default in a standard tiled wet area. They are not suitable for metals other than aluminium and stainless steel.

Neutral cure silicones — Wet Area Silicone, Roof & Gutter Silicone — cure without releasing acid. Choose them around mirrors, on stone, on a wider range of metals, and anywhere the acid would be a problem.

Cure rates, and why they matter more than you think

Sealants cure from the outside in, at a published rate per day. That means total cure time depends on how thick the bead is, and a skin is not a cure. Wet Area Silicone and Brick & Concrete Silicone both cure at about 2mm per 24 hours; 655FC at approximately 3mm per 24 hours; 3 in 1 at 3 to 4mm per 24 hours; 650FC at 2.5mm or more per 24 hours.

Questions answered in this section

• How long does 3 in 1 take to cure? and 3 in 1 cure time on tiles
• Wet Area silicone curing time, and whether a hair dryer helps
• Can you apply a second bead over the first?
• Does Wet Area Silicone stick to wood?
• The best sealant for caravan joints
• Sealing plexiglass to an aluminium frame
• Stopping water entering the base of a garden shed
• Sealing under coping on a fibreglass pool
• Adhering Perspex to wood

Safety: the sealants are Warning; the remover is Danger

A fifth question is worth adding to the four above, because it separates the products on this page more sharply than any performance figure does.

Product	Classification
3 in 1	Not classified as hazardous — no signal word, no hazard statements
Wet Area Silicone, Roof & Gutter Silicone, 655FC	Warning — H319 causes serious eye irritation, H317 may cause an allergic skin reaction
No Mould	Warning — H319, skin irritation Category 2, skin sensitisation Category 1
Brick & Concrete Silicone	Warning — H227 combustible liquid, H315 causes skin irritation, H317
Selleys Sealant Remover	Danger, Poison Schedule S5 — H304 may be fatal if swallowed and enters airways, H226 flammable liquid and vapour, H336 may cause drowsiness or dizziness, H315

Across the sealants themselves, the recurring statement is H317, and it is the one people dismiss. Skin sensitisation is not irritation. Irritation settles when contact stops; sensitisation is the immune system learning to react to the material — generally permanent once acquired, triggered thereafter by far smaller exposures, and worsening with each contact. It is built by repeated casual contact, and every method on this site ends the same way: tool the bead. Smoothing silicone with a bare wet finger, job after job, is exactly the exposure pattern that produces it. Tool with a spatula or a gloved finger. Cured sealant is inert.

The remover is a different order of product and deserves to be treated as one. It is more than 60% petroleum naphtha, and the classifications follow from that. H304 aspiration Category 1 means that if swallowed material reaches the airways it can cause chemical pneumonitis — which is why the first aid is never to induce vomiting. H226 with Dangerous Goods Class 3, Packing Group II means ignition is a real risk; and the rooms this product is used in are showers and kitchens, where a gas hot water pilot, a bathroom fan heater or a cooktop is usually within a few metres. H336 is what a 5mm layer left ten minutes, twice over, does in a small tiled room with the door shut.

So: ventilate actively rather than opening a door, remove ignition sources, wear nitrile gloves and eye protection, put the lid back on between applications and keep the tub out of reach of children — it looks like filler. And strip mechanically first. The remover is intended for a residual film under 5mm; cutting and scraping the bulk out keeps the quantity of solvent small.

Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand.

Common questions about sealants

Can I paint over a silicone sealant?

No. Complete all painting seven days before applying any Selleys silicone.

Can I put a new bead over an old one?

No. It may not adhere well to the original silicone. Cut the old bead out and lay a fresh one to the correct depth.

How long before the shower can be used?

No Mould is shower ready in 1 hour but must not be touched until full cure at 48 hours or more. Wet Area Silicone can take shower use 24 hours after application, with full cure at 72 hours.

Can I speed up curing?

Not with heat. Silicones need atmospheric moisture to cure, so a hair dryer has no effect on cure time and can hinder it.

What removes cured silicone?

Cutting and scraping, or Selleys Sealant Remover. Uncured product comes off with mineral turps.

Which sealant is safe on drinking water?

None of the wet area silicones — No Mould and Wet Area Silicone are both marked not suitable for food and drinking water contact. See is glass silicone safe for drinking water.

Can I apply sealant to a damp surface?

655FC can be applied to wet or damp surfaces with excellent adhesion, and 3 in 1 is suitable on wet or dry surfaces. The silicones need a clean, dry, oil-free surface.

Do polyurethane and silicone get on?

No. Neutral cure silicone may not cure properly where a polyurethane sealant has been used, and polyurethane sealants should not be applied in the presence of curing silicone. Keep the two apart.

More sealant questions

Browse more answered questions about sealing joints, selecting compatible products and preparing common surfaces.

• How To Fill Gaps Between Weatherboard and Door Frame
• How Long After Applying Wet Area Sealant Can It Be Exposed to Water?
• How To Seal an Exterior Gap in Concrete
• How To Seal a Toilet
• Joining Polycarbonate Sheeting
• How To Seal a Glass Stovetop to a Granite Benchtop
• Using Selleys All Clear on the Outside of a Wood Heater Flue
• Selleys All Clear New Improved Formula and Aquariums
• Best Sealant for Cracked or Broken Roof Tiles
• Silicone Putty Recipe