ORIGINAL: Spaceclam
where can you buy the stuff?
Why, at the small party store, of course!
Microballons are available at most hobby shops or online sources like Tower. The come in a "cottage cheese" type package... at least most of the time.
I like CaffenMan's description. When you go to mix them, if you have polyester resin, use that, it's easier to sand than epoxy or epoxy resin. I prefer to think of the resin as holding the microballons, rather than the other way around. Mix in microballons into a SMAll amount of resin until it is the consistancy of tooth paste. Don't try to use the mix where strength is required, it makes great fillets, etc. If you need strength, like fillets on a firewall, mix chopped fiberglass with the resin instead of mircoballons.