This thread is starting to bother me. Having built a fiberglass airplane, some of the techniques mentioned here are just flat out misguided. First off, you're building an
aircraft, not a boat where weight is not a concern. You want your fiberglass work to be
light. Bondo is heavy...Micro is light. Foam from Home Depot is heavy...aircraft grade foam is light. Just ask
Burt Rutan. Secondly, do you really want to save $5 on your $100,000 aircraft by using some unknown (and heavy) construction foam from Home Depot rather than the
light weight polyurethane foam readily available from Aircraft Spruce or other places? With the salmon-colored 2 lb./ cu. ft. foam from ACS, you can cut it with a bread knife and finish sand it by using a block of the same foam to work it. (Wear a mask!) ACS also sells more dense
Last-a-Foam for applications needing more compressive strength. (Sadly, the Clark Foam is no longer available.) Thirdly, when you're sticking two blocks of foam together, the adhesive is going to leave a hard glue line at the joint which will be impossible to sand out. (Disclaimer: I've never tried the Scotch 77 spray adhesive, so that might avoid that problem). My advice is to spend an hour to watch the Burt Rutan video linked above to learn the basics of fiberglass construction.