Which Selleys flexible gap filler do you need, and how do you choose?

A gap filler is a substance designed to fill the space in surfaces, or between two materials — the wall and the skirting board being the classic example. Gap fillers work with most building materials and can be painted over to suit your home, and they are used on plaster, concrete, brick, aluminium, wood, tiles and more.

What separates this category from the rigid wall fillers is one property: these retain flexibility once cured. That makes them the right choice for any surface where movement might occur. If you are filling a crack in the middle of a flat wall, you want a rigid filler. If you are filling the line between two building elements, you want one of these.

What this category covers

Gaps, cracks and holes around your home are not only unsightly — they let draughts and moisture into spaces where they do not belong, and they let insects in. Selleys makes flexible gap fillers for almost every type of surface and material, applied with a caulking gun.

Beyond the surface you are working on, the second thing to pay attention to is whether the filler is designed for indoor or outdoor use. That distinction runs right through the range and is the most common source of a wrong purchase.

How to choose

Interior work

No More Gaps Interior Multipurpose is the general interior filler, delivering a smooth finish with superior flexibility and low shrinkage. Low shrinkage is the property that decides whether you fill a gap once or twice — a filler that shrinks as it dries leaves a shallow trough down the middle of the bead that has to be filled again.

No More Gaps Interior Fast Dry is the fast-drying option, and Selleys notes that it can be used on interior and exterior surfaces. Choose it where you need to paint or move on quickly rather than wait.

Bathrooms, kitchens and humid rooms

No More Gaps Bathroom & Kitchen offers superior water and mould resistance and can be painted over to blend in. This is the product for the skirtings, cornices and frames in a room that gets steamy.

It is not for the gaps around a bath or sink, or inside a shower recess — those are sealing jobs handled by wet area silicone, not filling jobs.

Exterior work, where weathering matters

For superior resistance to weathering, Selleys specifies No More Gaps Exterior or No More Gaps Bricks & Mortar. Both were developed exclusively for outdoor use and offer weather and UV resistance without colour deterioration or staining.

That last clause is the practical reason to use an exterior-rated product outside rather than an interior one that happens to be to hand. Interior fillers are not formulated for sustained UV exposure, and the failure mode is not immediate — it is discolouration and staining that appears over a season or two, usually as a yellowed or dirt-streaked line where the filler was.

Where you do not want to paint afterwards

No More Gaps Coloured Gaps gives a seamless finish without the need to paint afterwards. Use it where the surrounding finish is already done and repainting would mean more work than the filling itself.

Timber floors and panels

No More Gaps Timber Floors is designed for filling gaps around and between tongue and groove timber flooring, and between other timber panels and fittings.

Large gaps and cavities

No More Big Gaps is the foam filler in the range, giving superior insulation and coverage for large gaps and cracks. Pro Trade Expanding Foam is the trade equivalent — a triple expanding, non-shrinking, multipurpose PU foam.

Foams belong to a different job from cartridge fillers. Where a gap is wide enough that a bead of filler would slump or crack, a foam expands to fill the void and is trimmed back afterwards.

Trade acrylic gap sealants

Pro Trade Gap Filler Fast Skin is an acrylic gap sealant formulated with a quality polymer and high solid content, giving extremely low shrinkage and excellent resistance to paint cracking. Pro Trade Gap Filler Smooth Finish is a white, paintable, flexible acrylic gap sealant suitable for interior and exterior applications.

Resistance to paint cracking is the specification that matters on trade work. A filler that stays flexible but whose surface splits under a paint film produces a hairline crack in the paint along the joint, which is the defect a painter gets called back for.

The products in this range

• No More Gaps Interior Multipurpose — smooth finish with superior flexibility and low shrinkage, for interior gap filling.
• No More Gaps Interior Fast Dry — fast-drying and easy to use, for interior gaps.
• No More Gaps Bathroom & Kitchen — superior water and mould resistance, paintable, for bathrooms and kitchens.
• No More Gaps Exterior — for exterior surfaces, developed exclusively for outdoor use with weather and UV resistance.
• No More Gaps Bricks & Mortar — for exterior brick and mortar gaps, weather and UV resistant.
• No More Gaps Coloured Gaps — seamless finish without needing to paint afterwards.
• No More Gaps Timber Floors — for gaps around and between tongue and groove flooring and timber panels.
• No More Big Gaps — foam filler giving superior insulation and coverage on large gaps and cracks.
• Pro Trade Expanding Foam — triple expanding, non-shrinking, multipurpose PU foam.
• Pro Trade Gap Filler Fast Skin — acrylic gap sealant with extremely low shrinkage and excellent resistance to paint cracking.
• Pro Trade Gap Filler Smooth Finish — white, paintable, flexible acrylic sealant for interior and exterior use.

The safety divide in this category — two very different products share the name

"Gap filler" covers two unrelated chemistries here, and they sit at opposite ends of the hazard range. Choosing between them on job suitability alone misses the more important difference.

Group	Products	Classification
Expanding polyurethane foams	No More Big Gaps, Pro Trade Expanding Foam	Danger. Eleven hazard statements, including H334 (may cause asthma if inhaled), H351 (suspected of causing cancer), H372 (organ damage from repeated exposure) and H362 (may cause harm to breast-fed children)
Acrylic gap fillers	The No More Gaps range, Pro Trade Fast Skin and Smooth Finish	Warning. A single statement, H317 — may cause an allergic skin reaction
Unclassified	No More Gaps Interior Multipurpose	Not classified as hazardous. No signal word, no hazard statements

The foams are the outlier, and by a long way. Both are 30–60% diphenylmethane diisocyanate (MDI). Respiratory sensitisation — H334 — means the immune system can learn to react to the vapour, after which the reaction is generally permanent, is triggered by concentrations far below the one that caused it, and worsens with each exposure. It is the mechanism behind occupational asthma in the spray-foam trades. The exposure window is while the foam is expanding and curing, in exactly the places these products are used: wall cavities, roof spaces, subfloors and window reveals.

If you are reaching for an expanding foam, ventilate actively, wear an organic vapour respirator rather than a dust mask, and keep others out of the space until it has cured and aired. Anyone breastfeeding should not use either product.

The acrylic fillers are a different proposition entirely. Their single H317 comes from isothiazolinone preservatives present at under 0.1% — the biocide that stops the tub spoiling. It is worth taking seriously, because sensitisation is permanent once acquired and builds from repeated casual contact, which is precisely what smoothing a bead with a wet finger over many small repairs amounts to. But it is one classification, not eleven, and it does not carry a respiratory, carcinogenicity or reproductive hazard.

If the hazard profile is the deciding factor — a nursery, a confined space, a household with someone asthmatic, or a job where you would rather not wear a respirator — No More Gaps Interior Multipurpose is the only product in this category that carries no hazard classification at all.

Comparing the fillers on the numbers that decide the job

Skin time governs how long you have to tool the bead. Movement capability governs whether the joint will survive seasonal expansion without splitting.

Product	Skin time	Movement capability
Coloured Gaps	3 minutes	±20%
Interior Fast Dry	6 minutes	Up to ±10%
Pro Trade Fast Skin	6 minutes	±10%
Bathroom & Kitchen	20 minutes	See product page
Exterior	30 ± 5 minutes	±25%
Timber Floors	30 minutes	25%
Bricks & Mortar	30 minutes, weather dependent	See product page

Two things fall out of that table that are easy to get wrong.

The fast products are fast in both directions. Coloured Gaps skins in three minutes and Fast Dry and Pro Trade Fast Skin in six. That is the point of them — you can paint sooner — but it also means a long run of skirting will have started to skin at one end before you have finished tooling the other. Work in shorter sections with those, and leave the 30-minute products for the long runs where you want the working time.

Speed and movement capability trade against each other. The three fastest fillers here carry the lowest movement figures (±10–20%), while Exterior and Timber Floors sit at ±25%. A joint that moves seasonally — an external expansion joint, a timber floor perimeter — wants the higher figure, even though it costs you working time. Filling a moving joint with a low-movement product is the usual reason a bead splits along one edge within a year.

Exterior also publishes a maximum gap size of 10 mm wide by 5 mm deep. Beyond that you are into backing rod territory, or into the expanding foams — with the hazard profile above attached.

Common questions

What is a gap filler, exactly?

It is a substance designed to fill the space in surfaces or between two materials, such as between a wall and a skirting board. Gap fillers work with most building materials and can be painted over.

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

Flexible gap fillers retain flexibility once cured, which makes them right for surfaces where movement occurs. Rigid wall fillers are for cracks and holes in flat surfaces that are not expected to move.

Which one can I use outside?

No More Gaps Exterior and No More Gaps Bricks & Mortar were developed exclusively for outdoor use and offer weather and UV resistance without colour deterioration or staining. No More Gaps Interior Fast Dry can be used on interior and exterior surfaces.

Which filler is best for a bathroom?

No More Gaps Bathroom & Kitchen, which offers superior water and mould resistance and can be painted over. For gaps around baths, sinks and inside showers, use a wet area sealant instead.

Do I need a caulking gun?

For the cartridge fillers, yes — Selleys describes applying its gap fillers with a caulking gun. The foam products are applied differently; check the individual product page.

What should I use for a gap that is too big for a bead of filler?

No More Big Gaps or Pro Trade Expanding Foam. Both are foams designed to fill large gaps and cavities rather than fine joints.

Will the filler crack when the paint goes on?

Pro Trade Gap Filler Fast Skin is specified for extremely low shrinkage and excellent resistance to paint cracking, which is the property to look for where the joint will be painted.

Can I use gap filler on tiles or aluminium?

Gap fillers can be used on plaster, concrete, brick, aluminium, wood, tiles and more. Check the individual product page for the surfaces that specific filler names.