RE: Microballoons?
I use both the phenolic (brown) and the glass (white) and both work very well. I use epoxy--hate the smell of the other stuff--and thouroughly mix the epoxy first then start adding the microballoons. Make a paste that is about like peanut butter in stiffness then it won't run when you apply it. I can't tell the difference in sanding, both sand quite easily. Both are available from Tower Hobbies and in most hobby shops.