Structural Foam for chassis stiffening

I am sure there are different foam compounds out there , but I have used a well known brand in aerosol cans in tight areas with little air exposure. At times it just turned into a goo and never set into foam. Other uses exposed to continual water it would absorb a lot of water over time. Just my experience, but because of that I would be very cautious to use that product or any similar unless proven that will not happen.
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