Selleys Ask an Expert: Tips for Using Flexible Fillers

What are flexible fillers, and which Selleys product answers which gap?

A flexible filler is a gap filler that stays elastic after it dries, so it can move with the building instead of cracking when the building moves. That is the whole point of the category. Timber shrinks and swells with humidity, a house frame settles, a metal window frame expands in the afternoon sun — and a rigid filler in any of those joints will craze, split or fall out within a season. This section of Ask an Expert collects the questions Selleys customers actually send in about those joints.

The practical decision is almost always the same three questions: is the gap inside or outside, does it need to be painted, and will it be wet? Answer those and the product choice usually follows.

Flexible filler or silicone sealant?

People use "gap filler" and "sealant" interchangeably, but on the Selleys range they are two different chemistries with different jobs.

No More Gaps products are water-based. They clean up with a damp cloth while wet, they are designed to be painted over, and they suit gaps around skirting boards, cornices, architraves, weatherboards and window and door frames. No More Gaps Interior Multipurpose has a movement capability of up to 15% (ISO 9046); No More Gaps Exterior carries ±25% (ISO 9046) along with UV and weather protection.

Silicone sealants are not paintable at all — the technical data sheets for the Selleys silicones state plainly that painting must be completed seven days before the sealant goes on. In exchange they tolerate far more movement and far more water. Brick & Concrete Silicone, for example, is rated at ±50% movement (ISO 9047, Method B).

The No More Gaps Exterior data sheet says it directly: where excessive joint movement occurs, or where the area sees frequent water immersion, use an appropriate Selleys silicone sealant instead of a water-based filler. If you want more background on the difference, types of gap filler and types of caulk cover it in more depth.

The gap size limit that catches people out

Both No More Gaps Interior Multipurpose and No More Gaps Exterior specify a maximum gap of 10mm wide by 5mm deep. This is not a marketing number — it is there because a deeper fill shrinks more as it dries, and shrinkage is what cracks the paint film over the top. Flat and ceiling paints crack first, because they contain less binder and flex less than a gloss or low sheen.

If your gap is wider or deeper than that, do not simply pack more filler in. Fill in layers, or move to a product built for the size. No More Big Gaps exists for exactly that case.

Questions answered in this section

• Filling the gap between aluminium window frames and adjoining timber frames
• Filling 3–10mm gaps between skirting boards and floorboards
• A clear product for the floorboard-to-skirting joint
• Filling brick gaps beside a swimming pool
• Whether No More Gaps can replace tile grout
• Removing No More Gaps from carpet

Safety: No More Big Gaps is a different chemistry, not a bigger tube

This section ends by sending you somewhere specific when the gap is past 10mm wide by 5mm deep: No More Big Gaps. That is not a larger cartridge of the same material, and the classifications are where the difference shows.

Product	Classification
No More Gaps Interior Multipurpose	Not classified as hazardous under GHS 7 — no signal word, no hazard statements
No More Gaps Exterior	Warning — H317, may cause an allergic skin reaction
No More Big Gaps	Danger — H222 extremely flammable aerosol, H334 may cause allergy or asthma symptoms or breathing difficulties if inhaled, H351 suspected of causing cancer, H372 causes damage to organs through prolonged or repeated exposure, H362 may cause harm to breast-fed children

H334 is the one to understand before you press the trigger. No More Big Gaps is an isocyanate polyurethane foam — MDI is 30 to 60% of the can. Respiratory sensitisation works like a skin allergy but in the airways: once the immune system learns to react, the reaction is generally permanent, is triggered thereafter by concentrations far below the one that caused it, and worsens with each subsequent exposure. There is no safe re-exposure afterwards.

The exposure window is while the foam expands and cures, not afterwards — cured foam is inert. That window falls in exactly the places this section's gaps live: skirtings, cornices, architraves, window and door reveals. Small, enclosed, indoors, usually with the house shut up. So the control is an organic vapour respirator rather than a dust mask, gloves, eye protection, and active ventilation rather than an open door. Keep other people out until it has cured and the space has been aired — particularly children and anyone with asthma.

H362 is unusual on a hardware product and deserves naming: anyone breastfeeding should not use it. H222 and the pressurised can mean no piercing or burning even when empty, no smoking while dispensing, and storage out of sunlight and below 50 °C.

The lower-hazard route is the one this page already describes. If the gap can be brought within 10mm by 5mm — by filling in layers and letting each dry — No More Gaps Interior Multipurpose does the job with no classification at all. Size the job down before you reach for the foam.

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

Common questions about flexible fillers

Can I paint over a flexible filler?

Over the water-based No More Gaps products, yes — that is what they are designed for. No More Gaps Exterior can be painted once it has skinned, roughly 30 minutes after application. Silicone sealants cannot be painted over at all.

How long before I can paint?

It depends on the paint, not just the filler. On No More Gaps Interior Multipurpose the published paint-over times are 0 minutes for semi-gloss, 10 minutes for gloss and low sheen, 100 minutes for flat and 120 minutes for ceiling paint. Oil-based paints need 24 to 48 hours.

What is the largest gap I should fill in one pass?

10mm wide and 5mm deep for both No More Gaps Interior Multipurpose and No More Gaps Exterior. Larger gaps need a different product or a layered approach.

Can I use an interior filler outside?

No. No More Gaps Interior Multipurpose is not suitable for gaps between tongue and groove boards or weatherboards — the data sheet directs you to No More Gaps Exterior for those.

What temperature can I apply at?

No More Gaps Exterior has an application temperature range of 5 to 40°C, and its limitations add that it is not to be stored or applied below 10°C. Interior Multipurpose should be applied above 10°C.

How do I clean up excess?

With a damp cloth, immediately, while the product is still wet. Once No More Gaps has cured it has to be cut and scraped off.

Will a flexible filler waterproof a shower?

No. No More Gaps Interior Multipurpose is not suitable for permanent water immersion or constantly damp areas, and even No More Gaps Bathroom & Kitchen directs you to Selleys Wet Area Silicone for gaps around baths, sinks and showers.

Do flexible fillers resist mould?

No More Gaps Exterior is described as mould resistant, and No More Gaps Bathroom & Kitchen contains a fungicide. Neither is a substitute for a silicone in a joint that is regularly wet.