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How to use expanding foam without emptying the can

Updated: 23 October 2026

How to use expanding foam is the fraction you leave empty, and the clock on the can you bought. The twelve cans in the [expanding foam comparison](/hub/fillers-adhesives-and-sprays/expanding-foam) do not share one drying time. This is the sequence for a straw can or a gun can. If the frame is uPVC or timber, read [expanding foam around windows](/hub/fillers-adhesives-and-sprays/expanding-foam/sub/expanding-foam-around-windows) before you fill. We used maker guides and amazon.co.uk notes from 19 October 2026, not a test rig. A qualifying purchase can earn a commission at no extra cost to you. Which can to hold is the [expanding foam filler buying guide](/hub/fillers-adhesives-and-sprays/expanding-foam/sub/expanding-foam-buying-guide).

How to use expanding foam is the fraction you leave empty, and the clock on the can you bought.

Our top picks

Soudal Soudal Soudafoam Gap Filler 750 ml

Best overall

Soudal Soudal Soudafoam Gap Filler 750 ml
8.7/10about £7.19
View offer
Gorilla Filler Gorilla Filler 500 ml

Rank 2

Gorilla Filler Gorilla Filler 500 ml
9.2/10about £6.75
View offer
Gorilla Gorilla Heavy Duty expanding foam 353 ml

Rank 3

Gorilla Gorilla Heavy Duty expanding foam 353 ml
7.4/10about £9.93
View offer
On this page
  1. 1.Before you fit the straw or the gun
  2. 2.How far to fill
  3. 3.When you can cut and paint
  4. 4.If the nozzle blocks or the can drips

Before you fit the straw or the gun

Clear dust and loose material. Foam needs a sound surface to grip, and Ferris reviews call it a poor choice on flaky faces. Mask brick, pipe and floor. Wear gloves, and ventilate. These are flammable aerosols. Do not store a can above 50 °C. Shake before you fit the straw or the gun. Penosil’s UK note says at least 20 times. Soudal’s Gun Gap Filler instruction says at least 20 seconds. Use the line on your can if it prints one. Mist the joint. Moisture helps a one-part polyurethane cure. Polycell’s 12-hour cut time is an example only, and Polycell is not a can in this set. If uncured foam lands on skin, wash it off. The Health and Safety Executive says wash skin and do not use solvents. Gun cleaner is for the gun, not your hands.

How far to fill

Straw foam should stop at about half the gap. Penosil puts it there because straw foam post-expands more, and you cannot meter it finely. Gun foam is about 60 to 70 per cent. Soudal’s Gun Gap Filler page says about 65 per cent. A hole that looks under-filled at one minute is often flush at twenty. A straw bead that packs a 15 mm gap around a 22 mm copper pipe swells out of the chase. Leave about half the depth empty. Overhead work drips. Gorilla Filler reviews mention drips when the gap faces upwards. Work the nozzle into the back of the void and pause. Halfway is not permission on a window frame. Frames need the right foam as well as a light bead.
Soudal Soudal Soudafoam Gap Filler 750 ml

Testsieger

Soudal Soudal Soudafoam Gap Filler 750 ml
8.7/10about £7.19
View offer

When you can cut and paint

The clock is on your can. The Soudal champagne twin says initial hardening in 25 minutes and a full cure of 24 hours. Bullets on a gun six-pack and one twelve-pack say cut within 25 minutes. Gorilla Heavy Duty says tack-free in 8 to 12 minutes and trim in 45 minutes. A stone-twin review waited about three hours, another 24 hours. One Gorilla Filler review said touch-dry within about an hour. Cut early and you drag uncured foam. Outdoor cured foam needs a coating where sunlight can discolour it. The champagne twin says Not Water Resistant. The single gun can, model 130585, says water resistant. Read the listing you bought.

If the nozzle blocks or the can drips

A straw can in this set is one use. Gorilla Filler reviews describe a nozzle that blocks after a few presses. Gorilla Heavy Duty reviews say the nozzle will not stay fitted. Paint Factory reviews describe a holed straw and a valve that emptied the can into the tube. If the straw blocks, do not keep squeezing. Wipe the tip, and if it will not clear, stop. You cannot park a started straw can until next week. A gun can waits only while it stays on a clean gun. Reviews on the single Soudal can report a leak when the gun is removed and refitted. Six-pack reviews say a clogged gun stays clogged unless you use cleaner. A metal gun does not turn Gap Filler into frame foam, and none of these cans is a specified fire stop.

Common questions

How full should I fill the gap?
About half with a straw can, and about 60 to 70 per cent with a gun can. Soudal’s Gun Gap Filler instruction is about 65 per cent. The foam keeps moving after you stop. A joint filled to the face will swell past it.
Should I wet the gap first?
Yes, a light mist. One-part polyurethane cures with moisture. Do not flood the chase. Dust and standing water both spoil the grip, so clean first, then dampen.
How long before I can cut it?
Use the time on your can. In this set that might be a 25-minute cut bullet, a 45-minute trim on the 353 ml Gorilla, or a 24-hour full cure on the champagne twin. A generic one hour will be early for one listing and late for another.
Why did the straw block?
Started straw tubes cure in the nozzle. Reviews on Gorilla Filler, Gorilla Heavy Duty and Paint Factory all describe blocked, loose or holed straws. A pause of a few minutes can be enough. Plan the bead so you do not stop in the middle.
Can I use the rest of the can next week?
Not a straw can, once you have pressed the trigger. The tube is the part that sets. A gun can can wait only if it stays screwed to a gun you then clean, and even then reviews report leaks after a refit. Do not plan a half can as stock.
Why is it dripping on an overhead pipe?
Gravity, plus more foam than the void can hold. Gorilla Filler reviews mention drips on upward work. Fill from the back, leave half the depth empty, and mask the pipe. Cutting the drip off later still leaves a stain if you waited too long to wipe the wet foam.

Related pages

Gun foam or straw foam

Best expanding foam

Expanding foam

Expanding foam around windows: the gap-filler limit

Expanding foam filler guide: straw, gun or neither

Sources

Public UK sources used for this comparison. amazon.co.uk prices, stars and review counts were taken from the listings on 19 October 2026.

  1. HSE isocyanates (retrieved 19 October 2026)
  2. Retained Regulation (EU) 2020/1149 (retrieved 19 October 2026)
  3. HSENI diisocyanates (retrieved 19 October 2026)
  4. Fire safety: Approved Document B (retrieved 19 October 2026)
  5. Hertfordshire Building Control PU-foam note (retrieved 19 October 2026)
  6. Soudal Soudafoam Gun Gap Filler (retrieved 19 October 2026)
  7. Soudal Soudafoam range (retrieved 19 October 2026)
  8. Penosil UK straw foams and gun foams (retrieved 19 October 2026)
  9. Polycell Expanding Foam (retrieved 19 October 2026)