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How to use a foam gun without filling the gap flush

Updated: 23 October 2026

You have a screw-on foam gun and a gun-grade can, and the joint is a window reveal, a door lining or a pipe chase. The mistake that wastes the can is fitting it without a shake, holding it the wrong way up, or packing the gap until it looks full. This is how to use a foam gun so the bead can still expand. It is not a ranking of the eleven guns in the [foam gun comparison](/hub/fillers-adhesives-and-sprays/foam-gun). Peter F. used public UK instructions from Screwfix, Penosil, Soudal and the Health and Safety Executive. A qualifying purchase can earn us a commission at no extra cost to you.

You have a screw-on foam gun and a gun-grade can, and the joint is a window reveal, a door lining or a pipe chase.

Our top picks

WORKPRO WORKPRO PTFE foam gun

Best overall

WORKPRO WORKPRO PTFE foam gun
7.7/10about £18.99
View offer
fischer fischer PUP M3 metal foam gun

Rank 2

fischer fischer PUP M3 metal foam gun
8.6/10about £19.84
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Lytool Lytool PU expanding foam gun

Best value

Lytool Lytool PU expanding foam gun
8.1/10about £14.29
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On this page
  1. 1.Before the can goes on
  2. 2.Fit the can and set the bead
  3. 3.How full to fill the gap
  4. 4.After the bead

Before the can goes on

Ventilate before you shake anything. The Health and Safety Executive says isocyanates in polyurethane foams can cause occupational asthma. Suitable gloves are part of the job. Nitrile is the example HSE gives. If foam lands on skin, wash with soap and water. Do not use solvent on your hands. Clear loose dust out of the joint. Penosil's UK guide puts the can at around 20°C. A cold can from a van spits gas. Bring it indoors and let it warm, without sitting it on a radiator or piercing the aerosol. Do not use this foam as a fire stop.

Fit the can and set the bead

Shake before the can meets the gun. Penosil says at least 20 times. Soudal sheets say at least 20 seconds. If the can prints its own line, follow that. Screw the gun on so the can ends on top. Screwfix describes the working position as holding the gun upside down: the nozzle points into the joint and the canister is above the basket. Set the rear flow screw small before you commit to the frame. If the basket leaks as soon as you fit the can, stop. What to check before you buy is on the foam gun buying guide.
WORKPRO WORKPRO PTFE foam gun

Testsieger

WORKPRO WORKPRO PTFE foam gun
7.7/10about £18.99
View offer

How full to fill the gap

The published figures disagree, so the can in your hand wins. Penosil, Soudal and Gap Filler C&F do not print one fill fraction for gun foam. Fill short of flush. A joint packed to the face can bow a uPVC or timber frame. If the gap is deep, lay a layer, let it cure as that can instructs, and add another rather than filling 80 mm in one pull. Straw cans are a different tool, on foam gun vs straw.

After the bead

Leave a part-used can on the gun if you will shoot again soon. Screwfix and Soudal both say that. Close the flow screw and wipe the tip. If you take the can off, Soudal says to empty it before you unscrew it, then clean the gun. The flush is on how to clean a foam gun. Do not pierce or burn leftover aerosols. Council rules differ. Expanding-foam cans often belong at a household recycling centre. Stop if the gap is a fire door, a flue or a fire-stopping line. None of the guns in this comparison is fire-rated.

Common questions

Which way up does the can go?
The can goes on top. You hold the gun so the nozzle points into the joint and the canister is above the basket, which Screwfix describes as holding the gun upside down. Fit it the other way and you are more likely to discharge gas than a controllable bead. Purge away from your face until foam, not propellant, comes out.
How many times do I shake it?
Shake before the can is fitted, then again during the job. Penosil's figure is at least 20 times. Soudal sheets say at least 20 seconds. Those are not the same instruction, so follow the can you are holding if it prints its own line. A short shake after the gun is already fitted is not a substitute for the first one.
How full should the gap be?
Short of full, and the percentage depends on the formula. Penosil says about 60 to 70 per cent for gun foam. Soudal's FAQ says at least 50 per cent for gun-grade foam, a gun-foam sheet says about two thirds, and Gap Filler C&F says 65 per cent. Straw and Genius fills are lower because those foams expand more. Read the can. Do not fill the joint flush.
Do I wet the gap or the foam?
Mist the joint before you shoot, so the surface is damp. Do not flood it, and do not spray water onto the bead to force a cure. Penosil's guide is a moistened joint, not a soaked one. Standing water in the bottom of a gap will push the foam out or leave a void. Wipe drips off uPVC before they cure.
Can I trim it after an hour?
Sometimes, and only if that can says so. Screwfix says some foams can be trimmed after one hour, and that a full cure can take up to 24 hours. If the blade sinks, wait. Cutting early pulls soft foam out and leaves a hole you then have to fill again. Paint the cured foam only when the can allows it.
Can I leave a started can on the gun?
Yes, if you will use it again soon. Leave it screwed on, close the adjustment screw, and wipe the tip. Soudal says that if you remove the can you should empty it first and then clean the gun. Taking it off and standing the gun in a shed is how the barrel cures solid. The cleaning page covers the flush.
When do I stop and call a professional?
Stop if the opening is a fire door, a flue or a line meant to be a fire stop, if the frame has already moved, or if the specification is a loft or cavity system. Also stop if you already react to isocyanates. Ordinary gap-filling foam and the guns in this set are not a tested fire seal. Overfilling can still bow a uPVC frame even on an ordinary gap, so leave the expansion room the can prints.

Related pages

Best foam gun

Foam gun

Foam gun buying guide: thread, cleaner and reviews

How to clean a foam gun before the barrel cures solid

Foam gun vs a straw: straw, Genius Gun or screw-on

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. HSE, occupational asthma substances (retrieved 19 October 2026)
  3. Penosil UK, straw foams vs gun foams (retrieved 19 October 2026)
  4. Screwfix, how to use an expanding foam gun (retrieved 19 October 2026)
  5. Soudal UK, storing an opened can (retrieved 19 October 2026)
  6. Soudal Gun & Foamcleaner TDS (retrieved 19 October 2026)
  7. fischer UK, PUP M3 33208 (retrieved 19 October 2026)
  8. Environment Agency aerosol canister guidance (retrieved 19 October 2026)