Exploring Caulking: The Ultimate Guide to Sealants

What is caulking?

Caulking is the process of sealing a joint between two materials so that water and air cannot pass through it. Caulk itself is a substance used as a sealant and filler in construction, and it aims to prevent the penetration of water and air between two materials.

It can be used in windows, bathrooms, kitchens and more, making a home water and airtight and increasing protection from wind, heat and moisture. Caulks can also be used to seal cracks when painting or staining materials.

Where what is caulk? covers the material, this page is about the process — where it belongs in a job, what it achieves, and how to decide when to do it.

Why very small gaps matter

Draughts, bugs and runoff water can enter your home through gaps as small as 1/64 of an inch — under half a millimetre, about the size of a straight pin.

That figure is the reason caulking is worth doing at all. Gaps at that scale are invisible from across a room and, added up around a house, they represent a continuous path for air, moisture and insects. You do not notice any one of them; you notice the draught, the mould, or the ants.

It also explains why caulking rewards thoroughness over speed. A seal that misses 200 mm at the end of a run leaks almost as much as no seal, because water and air find the gap you left.

What caulking achieves

Blocks draughts, which can save money on energy bills.
Keeps out water, including runoff around windows and doors.
Keeps out pests.
Seals cracks before painting or staining, so the finish reads as a continuous surface rather than a set of separate pieces.
Forms a bond between different types of building materials — timber to masonry, metal to concrete, tile to benchtop.

That last point is worth separating out. Caulk can be used anywhere to seal gaps or to form a bond between different types of building materials. Two dissimilar materials move at different rates, and the joint between them is the one that opens up first — which is why it is almost always a caulked joint rather than a filled one.

Caulking as part of a painting job

This is where most people meet caulking for the first time, and where the sequence catches them out.

Caulking cracks and gaps before painting is standard practice — it is what turns a set of separate mouldings into a single painted surface. For that job you need a paintable, water-based gap filler, and the products publish their own paint-over times: No More Gaps Interior Fast Dry is paintable in about 10 minutes with gloss and low sheen paints, 100 minutes under flat, and 120 minutes under ceiling paints. No More Gaps Bathroom & Kitchen skins in about 20 minutes and is paintable at 60 minutes with gloss, sheen and low sheen, or 180 minutes with flat and ceiling paints.

Two conditions apply across those data sheets. Apply to gaps no larger than 5 mm deep and 10 mm wide, at temperatures above 10 °C, and avoid applying paint too thickly — otherwise the paint film cracks over the joint as the filler dries. And allow 24 to 48 hours before painting with oil-based paints.

The reverse order applies to silicone. Silicone sealants are not paintable, and across the Selleys range their data sheets require painting to be completed 7 days before the sealant is applied. So in a bathroom refresh the correct sequence is: prepare, fill, paint, wait a week, then seal the wet joints with silicone. Getting that backwards is one of the more expensive mistakes in a DIY renovation.

Choosing when to caulk and when not to

Caulking is not the answer to everything, and knowing the boundaries saves wasted product.

Caulk a joint that moves. Two different materials meeting, an expansion joint, a benchtop against a wall.
Fill a hole that does not move. A nail hole or a dent in plasterboard wants a rigid filler that sands flush, not a flexible sealant that cannot be sanded.
Do not caulk a drainage path. Weep holes in window frames and gaps under a bottom plate are often there deliberately. Sealing them traps water instead of letting it out.
Do not caulk over live mould. An established colony can penetrate a new seal even where the new product contains mould inhibitors.
Do not caulk over old caulk. New sealant may not adhere well to cured sealant.

A note about fire

Caulking is sometimes described as adding protection from fire. That is true only of products specifically formulated and tested for it. Fire resistance is not a general property of caulk, and an ordinary silicone or gap filler used in a fire-rated wall or penetration does not make it compliant.

Where a fire rating is required, the product has to be a fire-rated sealant selected and installed to the relevant system requirements — see the fire rated sealants range, and take advice from the building professional responsible for the rating.

The basics of doing it well

Clean and dry the joint. Selleys directions almost universally require surfaces to be clean, dry and free from dust, grease and oil. Old sealant, soap film and flaking paint all have to go first.
Mask 5 mm either side where you want a neat line — and remove the tape before the product skins.
Cut the nozzle to the joint width, at 45 degrees. Smaller than you think.
Push the product into the joint rather than laying it on the surface. Depth of bond is what makes a seal survive movement.
Tool it promptly — most silicones want smoothing within 5 to 10 minutes of application, and the pack will say what to use.
Respect the cure time. Dry to touch is not cured; silicones cure at around 2 mm per 24 hours.

The full method is at how to use a caulking gun.

Common questions about caulking

What does caulking actually do?

It prevents the penetration of water and air between two materials, making a home water and airtight and improving protection from wind, heat and moisture.

How small a gap is worth sealing?

Draughts, bugs and runoff water can enter through gaps as small as 1/64 inch — under half a millimetre.

Do I caulk before or after painting?

Before, if you are using a paintable water-based filler. After — with at least 7 days between painting and sealing — if you are using a silicone, since silicones are not paintable.

Which product do I use for cracks I am about to paint?

A water-based gap filler. Check its paint-over time on the pack, and keep the gap within 5 mm deep and 10 mm wide to avoid paint cracking.

Can caulking really lower energy bills?

Blocking draughts is what saves the energy. Caulk closes the air paths that let conditioned air out and outside air in.

Does caulking keep insects out?

Yes — closing gaps where dust, air or insects may be able to enter is one of the stated purposes of caulking.

Is caulk fireproof?

Not as a general property. Only products specifically formulated and tested as fire-rated sealants provide a fire rating, and they must be installed to the relevant system requirements.

Can I caulk between two different materials?

Yes — that is one of the things caulk is for. Check the product's substrate list, since several exclude polyethylene, polypropylene and PTFE.

Should I remove the old caulk first?

Yes. New sealant may not adhere well to old cured sealant, and any mould underneath will come back through.

How long before the joint is watertight?

It depends on the product. Selleys No Mould is waterproof and shower ready 1 hour after application, with full cure at at least 48 hours. Wet Area Silicone cures in 72 hours. Read the pack.

Where should I be caulking around the house?

See where can you caulk? and how to prevent draughts in your home.

What is caulking and what is its main purpose?

Caulking is a fundamental home improvement technique used in construction and maintenance work that involves applying specialized materials—called caulks or sealants—into gaps, joints, and seams between surfaces. Its main purpose is to prevent moisture, water, and air infiltration through gaps and joints in building structures, protecting them from water penetration, air leakage, environmental exposure, and structural damage from moisture infiltration.

How does caulking work once applied?

Caulking works by extruding sealant materials into joints where they fill and seal the spaces between different materials. Once applied, the sealant remains flexible so it can stretch and accommodate structural movement, and it undergoes a curing process—typically taking approximately 48 hours to fully cure—resulting in a barrier that maintains its integrity under various environmental conditions.

What are the parts of a typical caulking gun?

According to the page's guide on using a caulking gun, a caulking gun typically has a barrel or cradle that holds the cartridge, a plunger rod, a large trigger for extruding the product, and a small trigger that releases the plunger rod to remove the cartridge.

How do you prepare a caulking cartridge before use?

To prepare the cartridge, cut the tip off (secateurs work well and are safer than knives), then cut the nozzle to the width of the joint or desired bead size at a 45-degree angle. Once done, insert the cartridge into the gun, making sure the plunger rod is pulled all the way out first.

What is the recommended technique for applying caulking product into a joint?

If it's your first time, practice by gunning the product onto some cardboard to get a feel for how it comes out. When ready, apply the product into the joint by gunning in a forward direction, pushing the product in as you go. Releasing the trigger to pull again may create a small blob, which can be smoothed out afterward when smoothing sealant, while glue will just get squished flat.

How should you finish off a caulked joint?

To finish off a joint, soapy water often works well, but the manufacturer of the product will recommend the best way to finish their specific sealants or fillers. Glues can be brought together and clamped in place until they dry.

How long does it take to complete the caulking gun process according to the guide?

The page's how-to guide for using a caulking gun lists a total time of 60 minutes (PT60M) for the process.

In what common areas of a building is caulking typically applied?

Caulking is commonly applied to joints between building materials (both vertical and horizontal), window and door frame perimeters for weatherproofing, exterior walls and cladding joints for weather sealing, construction and expansion joints to accommodate structural movement, and kitchen and bathroom areas where moisture resistance is critical.