Gary, FWIW, seems to me that there is also a Phenolic micro baloon product
available. Don't remember where I've seen it (and it could be one of those
"false memory" thingies that sometime befall some of us - ahem - older
folks) and haven't a clue as to their properties or uses. I believe it was
used strictly for a easily sanded light weight filler.

Dan Schaefer