I do not find the epoxy/microballoon mix difficult to either apply or to sand. Just use a slow epoxy, thirty minute or slower. First thouroughly mix the epoxy before adding the micro balloons. Add balloons, stir well, add more, stir some more etc. untill you have paste about as thick as heavy peanut butter. It will hold its shape then and will be quite easy to sand. You can feather the edges by dipping your finger (wear rubber bloves if you are alergic to epoxy) into alcohol and rubbing the edges to smooth and feather them. Let it cure well before sanding.