Types of Sealants Explained | Find the Right Sealant for Your Project

What are the different types of sealant, and which one does your job need?

A sealant is a flexible material piped into a joint or seam to stop air, water, dust and insects passing through it. Caulk is the same idea by another name — in the past everything from cast iron to mud was used for the job; today it is a flexible compound that can close anything from a hairline crack to a sizeable hole. The reason there are so many different sealants is that a joint around a shower tray, a joint in a concrete slab and a joint on an exhaust manifold are three completely different engineering problems.

The one thing worth understanding before you buy is that sealants are chosen by the conditions the joint sees, not by the room it is in. The three conditions that decide the product are how much the joint moves, how much water and UV it is exposed to, and how hot or cold it gets. Get those three right and the seal lasts; get one wrong and it fails, usually within a year or two.

The main families of sealant

Silicone sealants

Silicone is the default for wet areas, glass, roofing and general weather sealing. It stays flexible over a wide temperature band, sheds water, and bonds well to non-porous surfaces such as glazed tile, glass and metal. Silicones split into two cure chemistries that are not interchangeable:

• Acetic cure silicones release acetic acid as they cure — that is the vinegar smell over the first day. They bond strongly to glazed ceramic, glass and porcelain.
• Neutral cure silicones cure without releasing acid. Use these where the acid could matter — around mirror backing, natural and engineered stone, and sensitive metals. Selleys Wet Area Silicone, Glass Silicone and Roof & Gutter Speed Seal are neutral cure products.

Silicone's main limitation across the range is paint. Selleys' wet area silicones are not paintable, and their technical data sheets ask that any painting be finished seven days before the sealant goes in. If you need to paint over the joint, silicone is the wrong family and you want a paintable gap filler or a hybrid instead.

Polyurethane sealants

Polyurethane sealant is the product of a chemical reaction between a glycol and an isocyanate. It is resistant to corrosion, water and chemicals, flexible and heavy-duty, and it is what gets specified for structural and civil joints as well as DIY work. Selleys' polyurethane joint sealants give real figures worth knowing: Pro Trade Joint Sealant FC and Pro Trade Joint Sealant LM both carry ±25% joint movement capability, and cure at a minimum of 3mm and 2.5mm per 24 hours respectively at 23°C and 50% relative humidity.

Unlike silicone, these can be painted — water-based paints go straight over the cured sealant, and if you want a solvent-based topcoat you apply a water-based primer first. Priming the substrate is generally not required on common building materials, though a pre-test for adhesion is recommended.

Hybrid and SMP sealants

Hybrid chemistry sits between silicone and polyurethane. Selleys Storm Sealant is the clearest example of what these can do that silicone cannot: it is a moisture-curing silyl-modified polymer that forms an immediate waterproof seal on any building material, wet or dry, indoors or out — 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, elongation at break above 250% and ±20% movement capability. It takes two coats of quality water-based exterior paint; solvent-based paints will not work over it.

Roof sealants

Roof sealants have to be durable and water-tight because of where they sit — full sun, driving rain, and a substrate that expands and contracts all day. They are used to repair cracks and leaks or to reinforce joins between materials. Silicone is a powerful barrier here against both sun and rain. Selleys Roof & Gutter Silicone is the standard product, with excellent UV and weather resistance, a skin time of 5 to 10 minutes at 23°C and 50% relative humidity, a cure rate of 2mm per day and full cure in about 72 hours. Its published service temperature range is 0°C to 70°C — comfortably inside what an Australian roof sees, but worth checking against your own situation before you assume "roof" means "any heat".

Spray sealants

A spray sealant has a different texture again — it goes on as a foam or aerosol barrier that adheres to the material and sets into a solid protective layer. There are versions for small gaps and cracks through to large holes. Roof & Gutter Spray Seal is sprayed from about 30cm, in two thick coats with drying time between, and is rain-ready in roughly 2 hours with full cure in about 24 hours before painting. Pro Trade Expanding Foam expands to two or three times its volume, is touch dry in 15 minutes and fully cures in around 3 hours at 3 to 5cm depth.

The expanding foam carries a limitation that catches people out: it must not be left permanently exposed to sunlight. Paint, sealant, filler or another opaque material has to go over it or UV will break it down.

Concrete sealants

Concrete sealants protect the surface from water damage, oils, salt and other chemical contaminants, and are particularly worth doing on decorative concrete. They are typically sprayed on and act as a barrier between the concrete and the contaminant. For sealing joints in masonry rather than coating a slab, Selleys Brick & Concrete Silicone is the relevant product — it carries a ±50% joint movement capability and a service temperature range of -40°C to 140°C, which is why it suits expansion joints that open and close with the seasons.

Fire-rated sealants

Fire-rated sealants are a specialist category with tested performance, not a general-purpose product used near heat. Selleys' Fireblock range — Fireblock AC (acrylic), Fireblock PA (polyurethane modified acrylic) and Fireblock XT (SMP) — is tested in accordance with AS1530.4-2014 for up to 240 minutes, for perimeter seals, construction joints and service penetrations. If a joint has a fire rating attached to it, only a tested product will do, and the tested detail matters as much as the product.

Waterproofing: what "waterproof" actually covers

One of the main reasons sealants are used in construction is to waterproof a space by closing gaps and protecting surfaces. Most silicone, polyurethane and bituminous sealants are waterproof — but waterproof and immersible are not the same thing, and this is the single most common misreading of a sealant label.

Selleys Wet Area Silicone, for example, is designed for splash and moisture in bathrooms and kitchens and its data sheet states plainly that it is not suitable for permanent water immersion. Brick & Concrete Silicone is not for below-waterline or below-ground applications. If your joint will sit permanently underwater — a pond, a tank, the inside of a plumbing fitting — you need a product specified for immersion, not a wet-area sealant.

The other half of waterproofing is cure time. A sealant is not waterproof until it has cured. Wet area silicones need 72 hours for full cure and must not be immersed before that; Roof & Gutter Silicone reaches full cure in about 72 hours as well. Silicones of this type cure at roughly 2mm per day, so a deep bead takes proportionally longer than a shallow one.

Matching the sealant to the joint

Situation	What matters most	Family to look at
Shower, bath, sink, splashback	Mould resistance, water, not paintable	Wet area silicone (neutral or acetic to suit the substrate)
Windows, glazing, mirrors	Neutral cure, UV, adhesion to glass	Glass silicone
Roof, gutters, flashings	UV, weather, fast skin	Roof & gutter silicone or spray seal
Brick, concrete, expansion joints	High movement capability	Brick & concrete silicone, polyurethane joint sealant
Leak that needs stopping now, in the wet	Bonds to wet substrates	Hybrid / SMP (Storm Sealant)
Fire or acoustic rated penetration	Tested compliance	Fire-rated sealant only
Automotive gaskets and engine bay	High service temperature	High-temperature RTV silicone

Figures quoted here come from the products' technical data sheets and are indicative results, not a basis for preparing a specification. Check the current data sheet for the product and batch you buy.

How to apply a sealant with a caulking gun

Most of these products come in a cartridge, and the gun is where beginners lose confidence. Four steps cover it.

1. Understand the gun. A caulking gun has a barrel or cradle that holds the cartridge, a plunger rod, a large trigger that extrudes the product, and a small trigger that releases the plunger rod so you can take the cartridge out.
2. Prepare the cartridge. Cut the tip off the cartridge — secateurs do this well and are safer than a knife. Cut the nozzle to the width of the joint or the bead size you want, on a 45 degree angle. Insert the cartridge into the gun with the plunger rod pulled all the way out.
3. Apply the product. If it is your first time, practise on a piece of cardboard to get a feel for the flow. Then work forwards along the joint, making sure the product is pushed into the joint as you go. When you release the trigger to pull again you may get a small blob — that can be smoothed out at the tooling stage.
4. Finish the joint. Soapy water often works well for tooling, but follow whatever the manufacturer recommends for that product, since it varies.

There is a fuller walkthrough at how to use a caulking gun, and background on the terminology at what is caulking.

Common questions about types of sealant

What is the difference between caulk and sealant?

In everyday Australian use they mean the same thing — a flexible material piped into a joint or seam to stop airflow, water and dust. Caulk is the older word; sealant is the term used on most modern product labels.

Which sealant should I use in a bathroom?

A wet area silicone. Choose acetic cure for plain glazed tile and glass; choose a neutral cure product where the joint touches mirror, natural stone or engineered stone. See the wet area silicone range.

Can I paint over silicone sealant?

Not the wet area silicones — they are not paintable, and Selleys asks that painting be completed seven days before the sealant is applied. If the joint needs painting, use a paintable product such as Paint Over Sealant or a polyurethane joint sealant.

Which sealant can be painted?

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

What does joint movement capability mean?

It is the percentage of the joint width the cured sealant can absorb as the joint opens and closes without failing. Brick & Concrete Silicone is rated ±50%, the Pro Trade polyurethane joint sealants ±25%, Storm Sealant ±20%. A joint that moves more than the rating will tear the bead or pull it off the substrate.

How long before a sealed joint can get wet?

Wait for full cure. Wet area silicones need 72 hours and must not be immersed before then; Roof & Gutter Silicone is fully cured at about 72 hours; Storm Sealant should not be disturbed for around 48 hours. Roof & Gutter Spray Seal is an exception — it is rain-ready in roughly 2 hours.

Are silicone sealants waterproof enough for a pond or tank?

Not the general wet area products. Selleys Wet Area Silicone is not suitable for permanent water immersion, and Brick & Concrete Silicone is not for below-waterline or below-ground use. Permanent immersion needs a product specified for it.

What sealant works on a wet surface?

Storm Sealant. It delivers an immediate waterproof seal on any building material wet or dry, and can be applied in the rain. Smooth it off with a spatula dipped in turps or white spirit within 5 minutes, then leave it undisturbed for about 48 hours.

Do I need to prime the surface before sealing?

For the Pro Trade polyurethane joint sealants, priming is generally not required on common building materials, but a pre-test for adhesion is recommended. Whatever the product, the surface must be clean, dry and free of oil, grease, release agents, dust and loose material.

Can I use expanding foam outside?

Yes, but it must be covered. Pro Trade Expanding Foam should not be left permanently exposed to sunlight — paint, sealant, filler or another opaque material has to go over it to prevent UV deterioration.

What holds up to engine or exhaust temperatures?

A purpose-made high-temperature RTV silicone. Selleys Auto Fix Instant Gasket High Temperature Sealant is rated for continuous service from -40°C to 260°C with intermittent resistance to 343°C (ASTM C1246:2017). A bathroom or general-purpose silicone in that position will fail.

How do I get the old sealant out before I put new in?

Cut the bulk of the bead away, then soften the residual film with Selleys Sealant Remover and scrape it off. There is a step-by-step at how to remove old sealant.