Use micro balloons, they come in two flavors, a brown colored one made of a plastic substance and a white one made of fiberglass. Both work well and much better than cornstartch etc. First mix the epoxy (use 30 minute or slower) well than add the microballoons mixing well as you add. You can add enough until the results is about like stiff peanut butter. This mix also sands quite well, not like pure epoxy. If you are making fillits, you can wet your finger with alcohol and smooth out the surface while it is still wet.