You might like to try "Glasplies" in Exchange and Mart.
They do a closed cell foam that comes in bottles marked "Component A" and "Component B".
You mix them together and then have about 30 seconds to do whatever you need with it.
I guess you could "paint" it on to a surface before it expanded!!
Check with Glasplies - they give excellent advice.
The stuff itself seems to have a long shelf life. I bought some for a project about six years ago and never got round to using it. A couple of weeks ago I was looking for something to fill a bottle to use as a float for a Danbuoy and sloshed a bit of each component into the bottle. Bingo - after a minute or so I had a bottle full of foam (and enough spilled on the barn floor for three more bottles).
What I discovered was that within a few minutes of the foam setting it was friable and easily "worked" so I managed to push the aluminium rod though the centre of the bottle.
A week later I picked up the foam that had landed on the floor (tidyness is not my strong suit) and it is really tough!!