How To Fill a Gap Between Floorboards and Skirting Boards

What is the best clear product to fill gaps between floorboards and skirting boards?

For a clear finish, use Selleys 655FC Fast Cure Adhesive Sealant. It will not shrink, it stays flexible, and once cured it can be painted — with water-based paints only. An alternative is Selleys Roof & Gutter Silicone Sealant, which is available in a translucent colour, but that product cannot be painted at all.

The reason a clear product is worth the extra thought at this joint is that the skirting-to-floorboard junction is usually the one place in a room where two finished timber surfaces meet without a paint film to hide the join. A white filler reads as a white line along the floor. A clear one disappears into the shadow of the skirting.

Why 655FC suits this joint

655FC is a premium 100% clear flexible adhesive and sealant built on a moisture-curing silyl-modified polymer, and its key property here is that it stays clear. The data sheet calls out that it is non-yellowing, which is what separates a clear sealant that still looks clear in three years from one that has gone the colour of weak tea.

Technology	Moisture curing silyl-modified polymer (SMP)
Colour	Clear
Packaging	290mL cartridge
Set time	20 minutes (23°C, 50% RH)
Skin time	45 to 55 minutes
Tooling time	Approximately 25 minutes
Cure rate	Approximately 3mm per 24 hours
Application temperature	5 to 35°C
Service temperature	0 to 70°C
Shore A hardness	30 ± 5
Elongation at break	Approximately 130% (DIN 53504)
Slump	0mm (non-sagging)

Indicative data only, not intended for the preparation of specifications.

Two figures change how you work. The 25 minute tooling time is long compared with a silicone, so you can run a full wall length and then go back and finish it. And the 3mm per 24 hour cure rate means a bead pushed 6mm into the gap is still curing on day two — worth knowing before you drag furniture back across the floor.

Painting: read this before you buy

655FC is paintable, but only with water-based paints, and only after it has cured. Solvent-based paints cannot go directly onto it — you would need to apply a water-based paint first as a primer. If you are planning to finish the skirting in an oil-based enamel, that is a real constraint.

Roof & Gutter Silicone, the alternative Selleys names for this job, cannot be painted at all. Its data sheet requires painting to be completed at least seven days before the sealant is applied. Choose it only if you are not painting over the joint.

How to fill the joint

1. Vacuum the gap out. Dust and old paint flakes in the joint are what a clear sealant will bond to instead of the timber, and in a clear product every bit of that debris is visible.
2. Make sure both surfaces are clean and dry, and free of wax, grease and polish. Floor polish under a skirting is a common cause of a bead that lifts later.
3. Cut the nozzle to suit the width of the gap and fit it to the cartridge. Load a caulking gun.
4. Extrude the sealant into the gap, pushing it in rather than laying it on. A bead sitting on the surface has almost no depth of bond and will peel.
5. Tool the bead within about 25 minutes, before it skins at 45 to 55 minutes.
6. Clean up any excess while it is still uncured — mineral turps will take it off. Once cured, it has to be scraped, sanded, or removed with a silicone remover.
7. If you are painting, wait for the product to cure and use a water-based paint.

Limitations worth knowing

• Cannot be painted with solvent-based paints unless a water-based paint goes on first.
• Not suitable for polyethylene, polypropylene or Teflon.
• Not recommended on bituminous substrates.
• Not for permanent water immersion, or for continuous water exposure before it has cured.
• Application temperature 5 to 35°C. It extrudes and cures better above 5°C.
• Store below 30°C and use within six months of opening.

If you would rather match the timber than go clear

Selleys No More Gaps Timber Flooring is a flexible, sandable, stainable gap filler that contains real wood fibre, and its uses explicitly include skirting boards and architraves as well as floors. It comes in four timber-matched colours in 380g cartridges — Jarrah, Hardwood, Walnut and Baltic — each matched to a group of common Australian timbers. Allow at least 24 hours before sanding residue off the timber surface, and note its limitation: not for gaps wider than 3mm in trafficable joints, and not for exterior use.

Safety: the removal step is the hazardous half of this job

The products this page recommends are mild. 655FC carries the signal word Warning, with H319 (causes serious eye irritation) and H317 (may cause an allergic skin reaction). No More Gaps Timber Flooring carries Warning and H317 alone. H317 is skin sensitisation, not irritation — permanent once acquired, then triggered by far smaller exposures — and it is built by repeated casual contact, which is what tooling a long skirting run with a bare finger amounts to. Use a tool or a gloved finger for step 5.

The product with the serious classification on this page is the one named for cleaning up cured sealant. Selleys Sealant Remover is classified Danger, Poison Schedule S5, and is more than 60% petroleum naphtha:

• H304 — may be fatal if swallowed and enters airways (aspiration hazard, Category 1)
• H226 — flammable liquid and vapour; Dangerous Goods Class 3, UN 1133, Packing Group II
• H336 — may cause drowsiness or dizziness
• H315 — causes skin irritation

Everything about this job is at floor level, and so is the vapour. Hydrocarbon vapour is heavier than air: it sinks and pools rather than dispersing upwards. You are kneeling along a skirting line with your face perhaps 400mm from the work, in a room that is usually shut up because you are working on the floor. That is also the height at which a gas heater, a floor-standing fan heater or a pilot light sits. Class 3, Packing Group II is a real flammability rating, not a formality — take ignition sources out of the room, and open it up rather than just the door.

H336 is what a whole wall's worth of solvent paste does in a closed room. The method is a layer at least 5mm thick, left at least ten minutes, often applied twice. Come out for air between applications, and remember the mineral turpentine used for clean-up is itself a flammable solvent.

H304 is the one to take most seriously at home. It is a beige paste in a 375g screw-top tub that looks like filler, sitting open on a floor in a house with children. Put the lid back on between applications, keep the tub up high, and never decant it into another container. If any is swallowed, do not induce vomiting — that is the mechanism that gets a hydrocarbon into the lungs. Rinse the mouth, give a glass of water and seek medical advice immediately.

Lower-hazard route: cut and scrape the bulk of an old bead out first. The remover is designed to work on a residual film under 5mm, so the less you leave, the less solvent the room has to take. Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand.

Common questions

Will 655FC shrink as it dries?

No. It is a non-slumping, non-bubbling formulation with 0mm slump, and Selleys states it will not shrink.

Can I paint over it?

Yes, with water-based paints, after it has cured. For solvent-based paints you must apply a water-based paint first as a primer.

Can I paint over Roof & Gutter Silicone instead?

No. Roof & Gutter Silicone is not paintable; painting must be completed seven days before the sealant is applied.

How long until it is fully cured?

It reaches full strength at 24 hours, and cures at roughly 3mm per 24 hours, so total time depends on how deep the bead is.

What if the floor is still slightly damp?

655FC can be applied to wet or damp surfaces and still adheres well, which is unusual for a sealant. That said, dust and polish still have to be off the surface.

How big a gap can I fill?

For vertical or overhead joints the data sheet limits it to 20mm wide. A skirting joint is neither, but keep the bead sensible and push it into the gap rather than heaping it on top.

What size does 655FC come in?

A 290mL clear cartridge.

How do I clean up?

Mineral turps while the product is uncured. Cured product needs scraping, sanding, or a silicone remover such as Selleys Sealant Remover.

Is there a guide to this job?

Yes — how to fill gaps between skirting boards and walls covers the closely related joint on the wall side.