What are sealants for, and how do you pick the right one?

Sealants have been used in homes for as long as there have been homes, to protect the building and to keep it secure, safe and warm. Caulk is the material used to seal joints or seams in a building to stop airflow, water and dust. In the past everything from cast iron to mud was pressed into service; today caulk is a flexible substance that can be piped into anything from a tiny crack to a much larger hole.

Selleys' range of caulk and sealant products is deliberately wide, because the jobs are genuinely different from one another. There are long-lasting paintable products, more malleable ones, and sealants designed specifically for bathrooms. The choice is driven by the conditions the joint sees — how much it moves, how wet it gets, how hot it gets, and whether it has to be painted — not by the room it happens to be in.

Start with the three questions that narrow the field

How much does the joint move? Every joint moves a little as the building expands and contracts, and some move a lot. Sealants carry a published joint movement capability — the percentage of the joint width the cured bead can absorb without failing. Brick & Concrete Silicone is rated ±50%, the Pro Trade polyurethane joint sealants ±25%, and Storm Sealant ±20%. A joint that moves more than the rating will tear the bead or peel it off the substrate, no matter how well it was applied.

How much water is involved? Splash and humidity are one thing; permanent immersion is another entirely, and most wet area products are not rated for it. Selleys Wet Area Silicone is explicitly not suitable for permanent water immersion, and Brick & Concrete Silicone is not for below-waterline or below-ground use. If the joint will sit underwater, you need a product specified for immersion.

Does it need to be painted? This one decides the family on its own. Selleys' wet area silicones are not paintable, and their data sheets ask that any painting be finished seven days before the sealant is applied. If paint has to go over the joint, look at a paintable gap filler or a polyurethane joint sealant instead — those take water-based paint directly, and solvent-based paint over a water-based primer.

The families in brief

Silicone

The default for wet areas, glass, roofing and general weather sealing. Silicones cure by reacting with atmospheric moisture, at roughly 2mm per 24 hours, which is why a deep bead takes longer than a shallow one. They split into acetic cure products, which release acetic acid as they cure and bond well to glazed tile, glass and porcelain, and neutral cure products, which cure without releasing acid and are the safe choice around mirror backing, natural stone and engineered stone. Selleys Wet Area Silicone, Glass Silicone and Roof & Gutter Speed Seal are neutral cure.

Polyurethane

Formed by the reaction of a glycol and an isocyanate, polyurethane sealants are flexible, heavy-duty and resistant to corrosion, water and chemicals. They suit construction and expansion joints, floor joints and bonding to concrete. The Pro Trade joint sealants cure at a minimum of 2.5 to 3mm per 24 hours at 23°C and 50% relative humidity, and are paintable.

Hybrid and SMP

Hybrid chemistry does things silicone cannot. Storm Sealant forms an immediate waterproof seal on any building material, wet or dry, indoors or outdoors — it can be applied in the rain to stop a leak on the spot. Its published figures are a skin time of 15 to 30 minutes, a cure rate of 2.5 to 3mm per 24 hours and elongation at break above 250%.

Spray and foam

Spray sealants apply as an aerosol or foam that sets into a solid protective barrier, useful for holes and cavities rather than neat joints. Pro Trade Expanding Foam expands to two or three times its volume and is touch dry in 15 minutes, but it must not be left permanently exposed to sunlight — paint, sealant or another opaque material has to cover it or UV will break it down.

Fire-rated

A tested category, not a general product used near heat. Selleys' Fireblock range is tested to AS1530.4-2014 for up to 240 minutes in construction joints and service penetrations. Where a joint carries a fire rating, only a tested product and the tested detail will do.

Two guides worth reading before you buy

Types of sealants is the comprehensive walkthrough: what caulk is, where it is used around a home, the differences between the silicone, polyurethane, hybrid, spray, concrete and fire-rated families, and how to apply a cartridge with a caulking gun. Start here if you are choosing a product for a job you have not done before.

How temperature resistance affects silicone sealant performance covers the property that most often gets misread. It sets out the published service temperature ranges — Wet Area Silicone at -40°C to +150°C, Glass Silicone at -60°C to +150°C, Roof & Gutter Silicone at 0°C to +70°C, and the high-temperature automotive gasket sealant at -40°C to 260°C continuous — and explains why "weatherproof" and "heat resistant" are separate claims.

For technique rather than product choice, the caulking section covers using a caulking gun, what caulking is and where you can caulk.

Common questions about sealants

What is the difference between caulk and a sealant?

None, in practice. Caulk is the older word for the same flexible material piped into a joint or seam to stop airflow, water and dust.

Which sealant do I use in a bathroom?

A wet area silicone. Acetic cure suits plain glazed tile and glass; use a neutral cure product where the joint touches mirror, natural stone or engineered stone. See the wet area silicone range.

Can any sealant be painted?

Not silicone. Polyurethane joint sealants accept water-based paint directly and solvent-based paint over a water-based primer, and Storm Sealant takes two coats of quality water-based exterior paint but not solvent-based paint.

Are all sealants waterproof?

Most silicone, polyurethane and bituminous sealants are waterproof, but waterproof does not mean immersible. Check the product's own limitation before using it below the waterline.

How long does a sealant take to cure?

Silicones cure at roughly 2mm per 24 hours and typically reach full cure at around 72 hours. Wet area silicones must not be exposed to immersion until fully cured.

How do I remove old sealant before resealing?

Cut out the bulk of the bead, then soften the residual film with Selleys Sealant Remover and scrape it off. See how to remove old sealant and how to replace old silicone.

What can I use on a surface that is already wet?

Storm Sealant, which seals wet or dry building materials immediately and can be applied in the rain.