Tow'd, I was actually looking at some of that stuff at a Home Depot yesterday. It looks like it might just do the job.

I also checked out the foam sprays, and they didn't really have any that would spray in a fan spray pattern. Most are designed for draft leaks in houses, and are sprayed through a narrow tube. The can must be held upside down as well, so that would be almost impossible to do a inner roof. I didn't know about the fire hazard of it! The lizard skin would probably be a good way of sealing the metal against rust, but I can't see how it would insulate. Is it fairly thick?

Thanks all for the advice.