It is strong and will bond to everthing.
Try West or System 3 expoy. They sell fiberglass filler by the bag full. But you might try a fine baking flower. The finer the filler, then easier to sand.
Good points to it: It is slow setting so it allows you to play with it longer than Bondo. Its hard, its light, it smells good when you use flour.
Bad Points: It is rough to sand on. Its very brittle. When I build my boats, I typically apply a first layer, sand, second layer (to fill the chips), sand, and so on.