Hey Doug,

You have not given us any clue as to what you are trying to do, but
have you considered "micro-balloons" available for West System
epoxies, available at many places, but for sure at any marine store.
They would do what is sounds like you want.

See:
http://en.wikipedia.org/wiki/Glass_microsphere and

http://www.westsystem.com/ss/

Take care.

Brian Lawson,
Bothwell, Ontario.

ps...you might try asking for just a 1/2 cupful at any marine hull
repair place, just to try a bit.