Fiberglass chopped up into bits knits the epoxy together pretty good. For ding repair on wood, lightweight spackle is good, but takes forever to dry if it's too deep. Bondo has a "Deluxe" formula that sands pretty easily and is good for feathering edges, like where a layer of a FG patch ends. Microballoons mix well with all types of epoxy and the drier the mix, the easier it will be to sand. If you leave the mix runny, it is very self leveling and works well like that, too.