Spray foam will form a skin that is waterproof, however if this skin is punctured or dented it will start to absorb water and become mushy and unbelievably heavy. Also, it expands as it sets, and if it is confined it can easily blow out bulkheads or separate hull from deck. I don't have a precise picture of what you want to do, but I would be inclined to stick with the float bags.

: I just purchased a used fiberglass kayak. I am wondering if it is a bad or
: good idea to use spray foam insulation into the bow and stern and not use
: the air bags? The foam floats and is not very heavy. The kayak is all
: fiberglass. I am new to the sport and have only used the kayak once but
: love it and am looking forward to many long rides.
: Any suggestions would be great.

: I also want to paint it can you use spray paint? What kind? Color?
: Thanks
: Chris