On Saturday, 4 November 2017 13:38:45 UTC, Andy Burns wrote:
When I was using a lot of foam (replacing all windows/doors) I had a
decent foam gun, and kept it cleaned after each usage, no problems.

Then when I started using it less frequently it started to get "gummed
up" between uses, so I stripped it down, it was ok again for a bit, I
tried leaving it with the foam can attached rather than
stripping/cleaning, and that ended-up completely blocked, stripped it
down again, and after the next time, gave up on the gun.

Instead I bought a hand-held can and a bunch of replacement nozzles,
used it once, discarded nozzle, cleaned the spout of the can with
acetone then left it stored upright, indoors.

Just came to use some a couple of months later, went to shake the can
and "uh-oh that feels almost solid" gave it a vigorous shake anyway,
didn't really do any good, attach nozzle but when pressed down not even
a dribble would come out.

So it becomes expensive to use in small quantities ...

The foam is expelled by means of gas. The gas leaks out over time so the cans have a shelf life. Buy them when you need them. There are various sizes.

The foam sets by means of atmospheric moisture, so it won't set in the can.
If gas is lost, you just get a gummy liquid out. Good as a glue but no foam.