>Two problems come to mind with this approach.  First, making
>sure the stuff stays in the bag.  Second, making sure it
>doesn't *bow* the panels.  Your advice to do small spots,
>move on, and come back after the first area cures does make
>sense.

Biggest problem with this is the likelihood of rusting out the body panels. 
Trapping water/vapour between the foam and body panels is bad news. Any 
water accumulating near the edge of the foam will seep in, by freezing and 
expansion aided by panel flex and capillary action. Ziebart rust proofing 
fails this way as the guck hardens up and lifts off the surface. Good 
rustproofing sprays are non-hardening. 
        Regular fiberglass will pass vapour, hence the need for vapour 
barrier to keep it out of the area, summer heat drives the vapour back out 
however.  Any water leaks due to rust holes will guarantee more rust, 
especially since the area may be holding heat better and staying wetter most 
of the winter.  Any restoration work is hell with foam in place.  I'm 
sticking with FG, very cheap, light weight, easy to work, easy to throw 
away. Remember, aircraft are aluminium skinned and don't see much salt 
action. Foam is good there. My $0.02 worth.  

Tim Smith