For thickening epoxy, many people have thier tricks.

From what I've tried, and read:

Micro ballons; as stated above, and very popular! Light weight / easy sanding.

Milled fiber: this is essencially shredded fiberglass; small strands. While it'll thicken the epoxy, it'll also add lotsa strength. You're introducing a 'random weave' of fibers. Not nesseccarily 'light', nor 'easy' to sand. Mostly used to 'beef up' structures. sanding not reccomended..

Balsa dust: Yeah, the stuff you get when building! save it, and add it to the mix. as good as 'micro ballons', cheaper, but not as light. sands OK, but does need a sanding.. (leaves a 'goose bump' surface)

Baby powder: Same as 'balsa dust, but a 'baby fresh scent' when sanding!

PB&J: Although sticky, does better at secureing bread to bread than balsa (or ply) to balsa (or ply)!!



There's soon going to be a time, when along with 'Milled Fiber' and 'Micro Ballons', we'll be able to purchace "Buckey Balls" and "Buckey strands"!!