Just a follow up to an old thread where foam guns were discussed. Just used mine after about 10 months in the garage and the Screwfix special worked perfectly.
The conclusion that I have come to is that a partly used canister can safely be left on the gun for months but you have to make sure everything is completely sealed by tightening up the adjusting screw. I have had guns that have gummed up usually around the ball valve and have suspected the problem is not the gun but cans themselves not providing a moisture proof seal.
Richard