What is a gap filler, and how do you choose the right one?

A gap filler is a material you press into a crack or joint so that it seals, looks finished and stays that way. The catch is that "gap filler" covers several genuinely different products — flexible, foam, rigid and waterproof — and picking the wrong type is the reason so many filled cracks reopen within a year.

The single question that decides most of it is: does the gap move? A joint between a skirting board and a wall moves as the building expands and contracts, so it needs a flexible filler. A dent in a plaster wall does not move, so a rigid filler that sands flat is the better answer. Put a rigid filler in a moving joint and it will crack; put a flexible filler in a dent and you will not get a smooth sanded finish.

The second question is where: interior, exterior, or a wet area. Each has a formulation designed for it, and the differences are about weather resistance, UV resistance and mould resistance rather than marketing.

The guides in this section

• Where can you use gap filler? — the applications around a house, and how to get filler properly into a crack.
• Types of gap filler — flexible, foam, rigid and waterproof explained, with the Selleys products in each category.
• Best gap filler to use — recommendations by room and situation.

The short version, by location

Interior gaps — skirting boards, cornices, the join between wall and ceiling. Selleys recommends a long-lasting filler with flexibility, resistant to further cracking where movement may occur: No More Gaps Interior Multipurpose or No More Gaps Interior Fast Dry. Both handle a maximum gap of 10mm wide and 5mm deep, and both are paintable — the Fast Dry considerably sooner.

Bathroom, laundry and kitchen — areas that stay damp for prolonged periods need a filler that is flexible, mould resistant and paintable. No More Gaps Bathroom & Kitchen is designed for filling and sealing cracks near windows, cabinets, skirting boards and walls, with ±25% joint movement. Note its own limit: it is not recommended around bathtubs, showers and sinks — those need a waterproof silicone seal such as Selleys Wet Area Silicone.

Timber floors — No More Gaps Timber Floors contains wood fibres that give a timber-look finish and can be stained to suit. It also seals between timber panels and other fittings, such as kitchen cabinetry. Movement capability is 25% (ISO 9046).

Exterior — between windows and walls, on eaves, paving, fences, tiles and between particle board and fibre sheeting. Use something formulated to provide a weatherproof seal: the Selleys exterior range is designed to be weather, mould and UV resistant. No More Gaps Exterior offers ±25% movement (ISO 9046).

Brick and cement — No More Gaps Bricks & Mortar gives a mortar-like finish for repairs in brick and cement, pathways, or between brick and the roof.

Limitations worth knowing before you buy

• No More Gaps Bathroom & Kitchen — not for outdoor use or constant moisture exposure; not for polyethylene, polypropylene or Teflon; not recommended around bathtubs, showers and sinks.
• No More Gaps Exterior — not for sealing around bathtubs and kitchen sinks; maximum 10mm wide by 5mm deep; not for areas prone to frequent water immersion; not to be stored or applied below 10°C.
• No More Gaps Bricks & Mortar — not for areas of frequent water immersion, not for replacing mortar in construction, and not for moving cracks. Not to be stored or applied below 5°C.
• No More Gaps Timber Floors — not for exterior use; not for gaps larger than 3mm wide in trafficable joints; not for gaps between tongue and groove boards or weatherboards, where the Exterior product is the right one. Store above 5°C.
• Interior fillers — 10mm wide by 5mm deep is the stated maximum for both interior products. Beyond that, shrinkage during cure becomes visible and paint can crack, particularly with low binder paints.

Where the gap is genuinely large, that is a different product again — No More Big Gaps or an expanding foam rather than a caulk.

Safety: crossing the 10mm line changes the hazard, not just the product

Almost everything in this section is benign. No More Gaps Interior Multipurpose is not classified as hazardous at all. Its stablemates — Interior Fast Dry, Bathroom & Kitchen, Exterior and Timber Floors — are each classified Warning with H317 (may cause an allergic skin reaction) and nothing more.

The exception is the product named at the end of the limitations list, for when the gap is genuinely large. Selleys No More Big Gaps is an isocyanate-based polyurethane foam, signal word Danger, carrying H222 (extremely flammable aerosol), H229 (pressurised container, may burst if heated), H334 (may cause allergy or asthma symptoms or breathing difficulties if inhaled), H351 (suspected of causing cancer — suspected, not established), H372 (causes damage to organs through prolonged or repeated exposure) and H362 (may cause harm to breast-fed children), along with H332, H335, H315, H317 and H319.

So the 10 mm wide by 5 mm deep limit on this page is a safety threshold as well as a performance one. Below it you are using an unclassified or Warning-level water-based filler. Above it you are using the most heavily classified product on the site. That is worth knowing before you decide a 12 mm gap is "close enough to big".

H334 is the one that should change your behaviour. Respiratory sensitisation is not passing irritation — once acquired it is generally permanent, and afterwards much smaller exposures trigger it. Symptoms can be delayed by hours, so feeling fine while you work proves nothing. Big-gap work is by definition cavity work: voids behind linings, under floors, in roof spaces. Those are unventilated, and the product's own guidance is active ventilation rather than an open door, and not to work in a confined space at all.

H222 and H229 govern storage as much as use. An extremely flammable aerosol should not be discharged near a flue, downlight or heater, and a can must not be left above 50 °C — a car boot, a roof space or a sunny shelf all exceed that in an Australian summer. A rusted or damaged can can burst.

Organic vapour respirator when dispensing, nitrile gloves, goggles. Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand.

Common questions about gap fillers

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

In practice, a gap filler is optimised for a paintable, sandable finish in a joint that moves a little; a silicone sealant is optimised for a waterproof seal in a wet area and is generally not paintable. Around baths, showers and sinks you want the sealant, not the filler.

How big a gap can I fill?

The interior No More Gaps products state a maximum of 10mm wide and 5mm deep, and No More Gaps Exterior gives the same limit. Larger voids need a big-gap product or a foam.

Can I paint over gap filler?

Yes, on the No More Gaps range. Timing varies: Interior Multipurpose takes 60 minutes for gloss and sheen paints and 180 minutes for flat and ceiling paints, while Interior Fast Dry allows gloss at 10 minutes and semi-gloss immediately.

Why did my filled crack reopen?

Almost always because the joint moves and the filler does not. Use a flexible gap filler in any joint that moves, and save rigid fillers for dents and holes in solid surfaces.

Which filler do I use around a bath?

None of them. Use a wet area silicone sealant — No More Gaps Bathroom & Kitchen specifically states it is not recommended around bathtubs, showers and sinks.

Can I use an interior filler outside?

No. Exterior work needs a product formulated for a weatherproof seal, with weather, mould and UV resistance.

Do I need a caulking gun?

For cartridge products, yes. See how to use a caulking gun for the technique.