FlyingFever - To add to what Bear750 said, micro-balloons are sold by most hobby shops. Basically they are a very small glass bead that looks like baking soda. You can mix it with epoxy to make a paste that sands easily. It will still be harder than balsa, so you don't want to use it as a general filler. But you can vary the amount of balloons in the epoxy to get lighter, easier to sand and weaker or stronger, heavier and harder to sand.