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How to use expanding foam

Updated: 23 October 2026

The ruined job is a gap filled flush plus a straw that will not work in the morning. How to use expanding foam, for these one-component polyurethane aerosols, is a short sequence: protect yourself, moisten a dry joint, shake, fill short of the face, then cut when it has cured and cover anything daylight can reach. The [insulating spray comparison](/hub/diy-supplies/insulating-spray) lists the seven amazon.co.uk cans. This page does not pick one. Peter F. followed Polycell's UK label and Soudal's method. We did not foam a test joint. A purchase through our links can earn a commission at no extra cost to you.

The ruined job is a gap filled flush plus a straw that will not work in the morning.

Our top picks

Soudal Soudal Soudafoam Gap Filler

Best overall

Soudal Soudal Soudafoam Gap Filler
9.2/10about £7.19
View offer
Gorilla Filler Gorilla Filler 500 ml

Best value

Gorilla Filler Gorilla Filler 500 ml
8.6/10about £6.75
View offer
Gorilla Gorilla Heavy Duty 353 ml

Alternative

Gorilla Gorilla Heavy Duty 353 ml
8.4/10about £9.93
View offer
On this page
  1. 1.Before you press the valve
  2. 2.How far to fill
  3. 3.Cut, paint, and sunlight
  4. 4.When the can stops working

Before you press the valve

Read the valve before you shake anything. A straw can has a plastic nozzle. A gun-grade can, in this set only Soudal and Bond-It, will not run without a foam gun, and that gun is not in the box. Soudal's gun gap-filler sheet gives a can temperature of about +5 °C to +30 °C. Polycell's label calls this type of aerosol extremely flammable and pressurised, and it names MDI. Gloves and goggles stay on, the room is ventilated, and there is no flame. If the frame is a new uPVC window that is not yet packed and screwed, stop and read the windows and doors page. Foam is not the clamp.

How far to fill

Fill short, then leave it. Soudal's DIY page says fill about half the joint and mist between layers if you need a second pass. The gun gap-filler sheet says fill about two thirds. Polycell talks about expansion of about two and a half times. A hole filled flush will swell past the face. Work with the can upside down if the label says so. A gun meters a straw's coarse flow better. It does not cancel expansion after the foam has left the nozzle. The gun versus straw page is about control, not about a foam that stays put by itself.
Soudal Soudal Soudafoam Gap Filler

Testsieger

Soudal Soudal Soudafoam Gap Filler
9.2/10about £7.19
View offer

Cut, paint, and sunlight

Tack-free is not full cure, and the clock depends on the can. Gorilla Heavy Duty's listing says tack-free in 8 to 12 minutes and trimming in 45 minutes. That pair of times is for that 353 ml can only. Soudal's listing says full cure in one day. Bond-It says 24 hours. Polycell says cut or paint after about 12 hours. Penosil's FAQ says an early cut can start the expansion again. If the bead is still soft under a crust, leave the knife alone. Gorilla Heavy Duty says outdoor foam must be painted or coated because sunlight may discolour it. Soudal's sheet says cured foam is not UV-resistant. A waterproof word on the title does not replace that cover. Cutting a swollen face will not pull a bowed uPVC section back into square.

When the can stops working

Straw cans in this set are one session in practice. Paint Factory, Gorilla Filler, and Gorilla Heavy Duty reviews describe a tube that blocks after the first use even when foam remains in the can. You can sometimes clear a straw with acetone or nail-varnish remover. Leave a gun-grade can on the gun if you will return, and run cleaner through while the foam is fresh. Soudal points to polyurethane-foam cleaner for uncured foam. Cleaner and acetone can mark plastics and paint, so test a hidden area. Cured foam comes off by cutting, not by solvent and not by flame. Stop for a fire-resisting joint, spray foam on rafters, or a structural window or door.

Common questions

Should I wet the gap first?
Yes, if the joint is dry and the water will not freeze. Soudal and Polycell both tell you to moisten the surfaces, and Penosil repeats that with the freezing exception. A light mist is the idea. A gap that is running wet is not a better cure, and you still fill it short of the face. Follow the can if it gives a different moisture note.
Why did the nozzle block overnight?
Because foam left in a straw cures in the tube. UK reviews on Gorilla, Ferris, and Paint Factory all describe a blocked nozzle, sometimes within the first use rather than the next morning. Clearing with acetone is a reviewer workaround, not a promise the can will restart. Gun-grade cans fail at the gun joint instead, especially if you remove the can and refit it.
How do I get wet foam off my hands?
Better not to get it there: wear gloves before you start. Polycell points to acetone or nail-varnish remover on fresh spills, and says to test a hidden area. Soudal's cleaner is for uncured foam and can mark plastics and paint. Cured foam on skin is a medical question if it is widespread, not a solvent project. If you wheeze, stop and get help. Do not burn the foam off.
The bead dripped overhead. What should I change?
Slow down and support the joint. UK reviews on Gorilla Filler, the Gorilla two-pack, and Ferris all describe drips when the bead is above you. A board, a shorter bead, and a misted surface hold better than a second huge squirt into the same hole. A gun gives finer control than a straw. It does not stop the foam expanding afterwards, and it does not stick a bead that was too wet to hold.
Is it safe to burn the cured foam off?
No. Soudal's gun gap-filler sheet says never burn cured foam, and the Health and Safety Executive's isocyanate notes are about exposure, not about a tidy flame. Cut cured foam away once it has hardened, and keep the aerosol away from flames while it is still a pressurised can. Do not pierce the can to empty it.

Related pages

Insulating spray buying guide

Best insulating spray

Insulating spray

Gun foam vs straw foam: valve, reuse, and one hole

Expanding foam around windows: frames, packers, bowing

Sources

Public UK sources and the amazon.co.uk listings compared on this page. Prices stored 19 October 2026. Updated 22 October 2026.

  1. HSE: Isocyanates (retrieved October 2026)
  2. HSE: Isocyanate spraying (retrieved October 2026)
  3. RICS: Spray foam insulation consumer guide (retrieved October 2026)
  4. Commons Library CBP-10658 (retrieved October 2026)
  5. Hertfordshire Building Control: PU foam (retrieved October 2026)
  6. Soudal UK: Soudafoam Gun Gap Filler TDS (retrieved October 2026)
  7. Polycell: Expanding foam (retrieved October 2026)
  8. Soudal UK: Filling gaps and holes (retrieved October 2026)