RE: Microballoons?
There are two kinds of microballoons, the ones made of glass, and a kind made from phenolic plastic. I prefer the phenolic type for a few reasons. The phenolic type is lighter weight, I think they sand more easily, and they are less hazardous to your lungs when sanded. Sanding the glass type creates glass dust which is harmful to breath.
You can tell the phenolic type easily by their reddish brown color, as opposed to white for the glass.