The reason I edited my post with the question about a real-life comparison is that I didn't know whether the air would remain entrained in the mixture or whether it would be displaced by the glue. I found a reference to the bulk density of powdered talc at around 800 grams/litre, depending upon mesh size (40 lb per cubic foot) and a dry bulk density for microballoons at about a fifth of that amount (8 lb per cubic foot or 160g/L). Ignoring the fact that entrained air could be different (and assuming my references are correct) the microballoon mixture should be less dense than a talc mixture. Also assumes that they can be mixed at a similar adhesive to filler ratio. You have to consider the other properties as well - strength and sandability and paintability - to determine which would be better for your application. As I mentioned, I have no direct experience with talc.

I also discovered that there are two different types of microballoons. Some are made from silica glass and others are phenolic bubbles. The phenolic material appears to have an even lower bulk density.

Found another interesting comparison on fillers:
http://www.fiberglasssupply.com/Prod...s/fillers.html

Ross