I have started to use this stuff.
[link=http://www.hobbypeople.net/gallery/864206.asp][/link]

It is NOT like the spray foam you get at the hardware stores. During the expansion process, the foam has the consistency if soda fizz. once the expansion is complete, it sets. This way, you can get a complete fill in every void of the hull. I tested the foam first in a plastic bottle to see how it would react. As I said earlier, it was like soda fizz, very soft, and it doesn't push like the spray foam does. I then used it in my Mini-Cat. I cut an access, then poured it in. It worked perfectly! I sealed the access afterwords.

Here is the access I made after I closed it off.


Here's what it looks like after the foam is set.