We built an entire car body out of foam and then covered both sides with fiberglass and polyester resin. EXTREAMLY strong. I weigh 175 lbs and can stand on the front end with no cracking etc. The reason for the strength is due to covering both sides. Think about a steel I-Beam. The reason that is so strong is because steel is hard to stretch or compress. It bends easily but not so easy to stretch/compress. The I-Beam forces the outside lays to stretch and compress in order to bend the beam in two. Having foam covered in fiberglass (both sides) is like making a continuous I-Beam through out your body. Want to have some fun? Make a sample and see how hard it is to break.
Here is our car body. Don't laugh, we are not autobody engineers and autobody technicians
Just an electrical engineer looking for something different to do.
http://picasaweb.google.com/stinger3wheel/Stinger