Good news/Bad news...

I've used 2-part expandable polyurethane foam ( http://www.fibreglast.com ) for a variety of projects, with varying results. The main issue, as everyone has noted above, is that you've got to make sure that expansion won't wreck what you'r doing. Generally, I've used it in an application where I had a keel and some formers, (and no sheeting or planking) and so the foam could expand freely. Later, I sand it back to the level of the formers, and glass it. The way to do it in a tight application requires patience. Build up the foam in small batches, rather than trying to guess the amount and do it all at once. Here are some examples:

1. Built the interior of a huge (90-inch) fuselage plug with it. ( http://homepage.mac.com/mikejames/rc...lugmaking.html )

2. Built an engine cowling plug and two large nacelles here. ( http://www.nextcraft.com/b200_construction03.html )

3. Built another large cowling with it here. ( http://homepage.mac.com/mikejames/rc...t_cowling.html )

4. Built a large canopy plug with it, here. ( http://homepage.mac.com/mikejames/rc...ut_canopy.html )

5. Built a fuselage with it, and this one did NOT work out. ( http://www.nextcraft.com/j47_sportjet02.html )

One of the "secrets" seems to be that this material needs to be stabilized immediately after a day or so of curing. All the projects above that worked out were glassed immediately. The one ( #5 ) that didn't work out was one that sat too long, and because the foam continues to cure for several days, it warped the part, and I had to remove the foam and resort to traditional methods. The canopy plug on this same project was made with the foam, glassed immediately, and it worked out fine.

Like any process, you'll have better results if you do some tests on parts that don't matter, before attempting something serious.