What is caulking, and where do you start?

Sealants have been used in homes forever, to protect and bolster the security, safety and warmth of a property. Caulk is a material used to seal joints or seams in buildings to stop airflow, water and dust. In the past, everything from cast iron to mud has been used as a sealant. Today, caulk is a flexible substance that can be piped into place to close gaps ranging from tiny cracks to bigger holes.

The Selleys range of caulk and sealant products is diverse, and suits a wide range of applications and projects — from long-lasting paintable products, to more malleable ones, to caulk designed specifically for bathrooms.

This section is a set of beginner guides. If you have never run a bead before, read them in the order below.

The guides

What is caulk?

The starting point. What caulk is, what it does — thermal insulation, moisture protection, keeping out dust and insects — and where it belongs in a home. Read this if you are not yet sure whether caulk is what you need.

What is caulking?

Caulking as a process rather than a material: sealing cracks when painting or staining, blocking draughts, moisture and pests, and how caulk makes a home water and airtight.

Types of caulk

The categories — silicone, latex, expandable and butyl-rubber — what each is good at, and how drying and curing times differ between them. Read this before you buy.

Where can you caulk?

Windows, wet areas, exterior doors, wood panelling and drywall, and why damp areas need a mould resistant, watertight product rather than a general purpose one.

How to use a caulking gun

Four steps to a professional finish: understanding the gun, preparing the cartridge, applying the product, and finishing the joint. Plus what to do with the leftover product so it is still usable next time.

The three decisions behind every caulking job

Whichever guide you start with, the same three questions determine what you buy and how the job turns out.

1. Is the joint wet, or just indoors?

This decides the product family. Damp areas like kitchens and bathrooms need a sealant that is mould resistant and watertight — a silicone-based product. A gap around a skirting board is a different job, and a water-based gap filler that can be painted over is usually a better answer there.

The distinction is written into the products themselves. Selleys No More Gaps Bathroom & Kitchen, for example, is a water-based, paintable filler formulated for damp and humid areas with a fungicide to protect against mould — and its own technical data sheet still directs you to use a wet area silicone sealant for gaps around baths, sinks and showers where a genuinely waterproof seal is needed. Water resistant and waterproof are different specifications.

2. Does the joint need to be painted?

This is the single most common regret. Silicone sealants across the Selleys range are not paintable, and their data sheets require painting to be completed 7 days before the sealant is applied. If you seal first and paint afterwards, the paint will not hold on the bead.

Where a joint must be painted, the answer is a water-based filler rather than a silicone. No More Gaps Interior Fast Dry is paintable in about 10 minutes with gloss and low sheen paints, and No More Gaps Bathroom & Kitchen skins in about an hour before painting.

3. How much will the joint move?

A gap between two materials that expand at different rates — timber and masonry, metal and concrete — moves every day. A rigid filler in a moving joint cracks. Sealant data sheets publish a joint movement capability for exactly this reason: most of the silicone range is rated at ±25%, and Brick & Concrete Silicone at ±50%.

An expansion joint, by definition, is a gap left to allow expansion to occur in a structure. Those are always filled with a flexible sealant, never a rigid one.

Two numbers worth understanding before you start

Skin time is how long you have to tool the bead after applying it. For most Selleys silicones it is short — 5 to 10 minutes is typical, and No Mould's is 20 minutes. Once a skin has formed, smoothing tears the surface rather than shaping it, and there is no recovery short of cutting the bead out.

Cure rate is how long until the sealant has its strength, and it is a different number entirely. Silicone sealants generally cure at around 2 mm per 24 hours, working inwards from that skin. A bead pushed 6 mm into a joint is still curing three days later. This is why "it feels dry" is not the same as "it is ready".

Where to go from here

• Sealants — the full range, sorted by application: wet area, construction, roofing, glass, multi-purpose, fire rated and auto.
• Gap fillers — the paintable, water-based side of the range.
• Which gap filler to use.
• Types of sealants.
• Product selector — narrow the range down by what you are sealing.

Common questions about caulking

What is caulk, in one sentence?

A material used to seal joints or seams in buildings to stop airflow, water and dust — an elastic substance that can be moulded into crevices.

Is caulk the same as sealant?

The words are used interchangeably for the same class of material. Selleys' own range is described as caulk and sealant products.

Which guide should I read first?

What is caulk? if you are starting from scratch, or how to use a caulking gun if you already know what you are buying.

Do I need a caulking gun?

For a cartridge, yes. Smaller tube packs are applied by hand, but a gun gives far better control on any run longer than a metre.

What is the difference between a sealant and a gap filler?

Broadly, sealants are flexible and waterproof but generally not paintable; gap fillers are water-based, paintable, and suited to interior joints that are not permanently wet.

Can I paint over caulk?

Over a water-based gap filler, yes, after its stated paint-over time. Over a silicone sealant, no — painting must be completed 7 days before the sealant goes on.

How long does caulk take to dry?

Caulks and sealants are typically dry to touch within about half an hour, but need considerably longer to cure and set fully. Each product differs — check the pack.

Which caulk for a shower?

A wet area silicone. See the wet area silicone range.