That air-blockage thing was a big problem for me the other month when I was working at Silo-X. I made some missiles, and used greatstuff as filler to keep the funnels (nose cones) on the PVC pipe (rocket bodies). But the foam remained uncured in the center because I'd sealed off the air. When I came back the next day, the sun beating down on them had reactivated the unfoamed stuff and foamed it, and all my nose cones had popped off. I had to take them off and carve the foam down and reglue the cones on. It's temperamental.
Krough had the right idea. 2-part urethane foam is chemically catalyzed, you don't need all that air getting to it to make it foam and cure. And you can mix up exactly the amount you need, no waste.