This is the first project where I really care about getting a good smooth surface. I narrowed my tank and needed just a bit of body filler in a couple of dings. Went to sand smooth and got some very small indentations I guess where there was an air bubble or something. Do you keep sanding till that comes out? do you add just a bit more filler on top... that doesnt seem like it would adhere well. Or do you grind it all out and start over?