1. Fillers are added to liquid resins and paint to lighten the finished product or to create a desired surface.
2. When used with epoxies, be sure to activate the resin before adding fillers. So, you do not throw off the ratio of the epoxy system or it will not work properly.
3. Talc and Fumed Silica will give you a nice creamy putty, but they are not very sandable.
4. Microballoons will give you a lighter and more sandable putty.
5. Phenolic Microballoons are the largest sphere and easiest to sand.