Ive always thought that the glass ones are called Colloidial Silica, and that the microballoons are made of plastic of some sort. They make a very sandable fillet, and the glass is quite a bit harder and heavier. I'd like to see a good discussion on various fillers and thier properties. Lots of folks using carbon now, cartridge toner, flour. What about colors? Is there a good blue and green out there? Ive used several different kinds of sawdust with interesting results.

> I believe that microballoons are very small glass spheres that are added
> as filler to the epoxy to increase the viscosity.