Choosing the Right Gap Filler: A Comprehensive Guide

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

Gap filling looks like a simple concept, but the range of fillers and putties is wide and each type has qualities that suit it to a particular situation. The most important factor in choosing correctly is where you will be using the product — and, closely related, whether the gap moves.

There are four broad families: flexible, foam, rigid and waterproof. Everything else is a variation within them.

Flexible gap filler

A flexible gap filler expands and contracts with the subtleties of your building, so no further cracks form. Flexible fillers exist for both internal and external use, and they are most often used on skirting boards and wall joins.

This is the family to reach for whenever two building elements meet. A wall and a skirting board are separate things that move independently, however solidly they appear to be fixed; a rigid material bridging that joint has to fail somewhere, and it fails as a hairline crack down the middle.

The Selleys flexible range:

• No More Gaps Interior Multipurpose — smooth finish, superior flexibility, low shrinkage. Paint over at 60 minutes for gloss and sheen, 180 minutes for flat and ceiling paints. Maximum gap 10mm wide, 5mm deep.
• No More Gaps Interior Fast Dry — the same job with a much shorter wait: semi-gloss immediately, gloss and low sheen at 10 minutes, flat at 100 minutes, ceiling paint at 120 minutes. Same 10mm × 5mm maximum.
• No More Gaps Coloured Gaps — for a finish that does not need painting afterwards.
• No More Gaps Timber Floors — for gaps around and between tongue and groove timber flooring, and between other timber panels and fittings. 25% movement capability (ISO 9046), stainable and paintable.
• No More Gaps Exterior — ±25% movement (ISO 9046), weather resistant.
• Selleys N-Mastic — a strong, flexible bond for sealing gaps and joints.

Foam gap filler

Foam fillers are designed for larger holes or gaps. Big cracks or gaps in walls let pests, dust and draughts into the house, and sealing them protects against further damage. Many of the Selleys foam fillers are expandable — they dry and cure into a semi-rigid seal, filling a void far larger than a caulk could bridge.

The relevant products are No More Big Gaps and Pro Trade Expanding Foam. Use them where a flexible caulk would simply disappear into the cavity.

Rigid fillers

As the name suggests, rigid fillers fill spaces in solid surfaces and dry hard. They can be smoothed over and sanded flat, which makes them the right choice for the walls and ceilings of your property — dents, nail holes and cracks in plaster, where the substrate is one continuous material rather than a joint between two.

The Selleys rigid range:

• Spakfilla Rapid — ready-mixed, lightweight, fast-drying, virtually non-shrinking and non-cracking, needing only one application. Sand and paint in 30 minutes for applications under 2mm; 24 hours for anything deeper. Moisture and mould resistant, interior and exterior.
• Spakfilla Heavy Duty — for larger gaps and cracks in walls and ceilings.
• Spakfilla Colour Smart — dries white and sands easily.
• Spakfilla Wall Repair Kit — an all-in-one kit for holes and cracks in interior walls up to 8cm wide, containing a self-adhesive wall patch, sanding block, spatula and a 100g tub of Spakfilla Rapid.
• No More Cracks Plaster Filler — dries quickly and sands easily with minimal shrinkage.

Rigid filler limits worth knowing: Spakfilla Rapid should not be used on plastic, metal, or exterior timber likely to swell and shrink as it absorbs water. Avoid using it below 5°C or in wet conditions. Do not nail into it, and do not use it for load bearing applications or anywhere load or compression occurs — behind a door where the knob touches the wall, for instance, where the filler will simply be crushed. Where movement occurs, use No More Gaps instead.

Waterproof and moisture resistant fillers

Water-resistant fillers are the solution for bathrooms, kitchens and windows. The Selleys range of water-resistant fillers is formulated to be flexible, with high adhesion, and to protect against mould growth.

No More Gaps Bathroom & Kitchen offers superior water and mould resistance with ±25% joint movement. Its stated limits are important: not for outdoor use or constant moisture exposure, not for polyethylene, polypropylene or Teflon, and not recommended for use around bathtubs, showers and sinks — those need a waterproof silicone seal such as Selleys Wet Area Silicone.

For genuinely wet repairs, Knead It Aqua is the waterproof, mouldable epoxy putty for plumbing and underwater repairs.

Wood putties

Wood putties are a category of their own — for filling holes in timber and hiding nail heads, where you want the repair to take stain or finish like the surrounding wood.

• Selleys Woodfilling Putty — fills cracks and holes for a smooth finish.
• Selleys Plastic Wood — easy to use, dries quickly, for interior wood repairs.
• Selleys Special Putty — a linseed oil-based putty for marine, automotive and industrial repairs.

Epoxy putties

The Knead It range is a different technology again: two-part epoxy putties you knead together and mould into place, which set hard and take load in a way a caulk never will.

• Knead It Multipurpose — general DIY repairs.
• Knead It Aqua — waterproof, for plumbing and underwater repairs.
• Knead It Steel — a fast setting, non-rusting co-extruded epoxy repair system for metal items.
• Knead It Autofix — for car repairs.

Specialist fillers

Selleys Fireproof Cement is for high-risk areas such as fireplaces and chimneys. Do not substitute a general filler in those locations.

Choosing in one sentence

Does the gap move? Use flexible. Is it a hole in a solid surface? Use rigid. Is it bigger than a caulk can bridge? Use foam. Is it wet? Use a water-resistant filler — unless it is actually around a bath, shower or sink, in which case use a silicone sealant.

Safety: the foam family is not like the rest of this page

Across the four families above, the hazard profiles are not comparable. One family is in a different class entirely, and choosing correctly is partly a safety decision.

Family	Example	Classification
Foam	No More Big Gaps, Pro Trade Expanding Foam	Danger. H222, H229, H334, H351, H372, H362, H332, H335, H315, H317, H319
Flexible, interior	No More Gaps Interior Multipurpose	Not classified as hazardous. No signal word, no hazard statements
Flexible, exterior	No More Gaps Exterior	Warning. H317 only

Both foams are isocyanate-based polyurethanes, and they carry the longest hazard lists of any product in the Selleys range. Three entries deserve naming properly.

H334 — may cause allergy or asthma symptoms or breathing difficulties if inhaled (respiratory sensitisation, Category 1). This is the lung equivalent of a skin allergy: the immune system learns to react, the change is generally permanent, and afterwards an exposure far below the one that caused it is enough to provoke wheeze or an asthma attack. It develops from repeated exposures that feel like nothing at the time.

H351 — suspected of causing cancer (Category 2). Suspected, not established; that is what Category 2 means, and it is worth stating precisely rather than overstating. H372 — causes damage to organs through prolonged or repeated exposure. Neither describes one careful can used outdoors. Both describe a pattern of small unventilated exposures repeated over years.

Connect that to where this page sends foam. The reason to reach for a foam is a void too big for a caulk — which in practice means a wall cavity, a subfloor, a roof space, the gap behind a skirting or around a pipe penetration. Those are enclosed, poorly ventilated, and they are where your face ends up closest to the work. That is the H334 exposure pattern exactly. Use forced ventilation or work in stages with breaks outside the space, wear gloves and eye protection, and treat a respirator as a reasonable purchase rather than an over-reaction.

Two more that follow from the packaging. H222, extremely flammable aerosol, and H229, pressurised container, may burst if heated — so no downlight, flue, heat gun or soldering near a fresh fill, and no cans left in a hot car or roof space. And H362, may cause harm to breast-fed children: if someone on the job is pregnant or breastfeeding, they should not be the one running the foam.

The alternative is on this page already. If the gap is within 10 mm wide by 5 mm deep, the flexible family does the work, and No More Gaps Interior Multipurpose is not classified as hazardous at all. Foam is for what a caulk genuinely cannot bridge — not for a gap you could have caulked.

Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand.

Common questions about types of gap filler

What is the difference between a flexible and a rigid filler?

A flexible filler expands and contracts with building movement so no new cracks form. A rigid filler dries hard and can be sanded smooth, which suits dents and holes in a solid surface but not a moving joint.

When should I use expanding foam instead of a caulk?

When the gap is large enough that a caulk would disappear into it. Foam fillers are designed for larger holes and gaps, and many of the Selleys foams expand and cure into a semi-rigid seal.

Can I sand a flexible gap filler?

Rigid fillers such as Spakfilla are the ones designed to be smoothed and sanded. Flexible fillers are tooled to a finish while wet rather than sanded afterwards.

Which filler is safe to use on timber that will be stained?

No More Gaps Timber Floors contains wood fibres and can be stained, and the wood putties are made for taking a finish. General wall fillers are not.

Can I use Spakfilla outside?

Spakfilla Rapid is rated for interior and exterior use, but it must be protected from rain until dry and painted over with two coats of paint, and it should not go on exterior timber likely to swell and shrink with water.

Can I nail or screw into filler?

No. Spakfilla Rapid's limitations state do not nail, and it is not for load bearing applications.

What do I use where a door knob hits the wall?

Not a rigid filler alone — the product may be crushed where load or compression occurs. That is a case for a patch and a hard-wearing repair, or a door stop.

What is the difference between a putty and a filler?

Broadly, putties are mouldable by hand and often used for timber holes or specialist repairs, while fillers are applied with a spatula or caulking gun. Epoxy putties like the Knead It range set hard and can take load; caulk-type fillers cannot.

Is there a filler I do not have to paint?

No More Gaps Coloured Gaps is made for filling gaps without needing to paint afterwards.

What should I use around a fireplace?

Selleys Fireproof Cement. A general-purpose filler is not rated for that environment.