well... what I would do, is get a can of black primer, and a can of grey primer, get a scrap of foam, and stickyspray some 60 grit to one side, shoot grey primer, wait to dry, then black primer, at first you probably won't have to do that even, you can just shoot the black. use the foam block to sand it, don't use a loose piece of sandpaper, the block will hit the high spots, and miss the low spots, and the black primer won't get touched in the low spots, so you'll see black areas that are low, fill with bondo (use dyna gold, bondo will ruin your sandpaper) using a new or well cared for plastic spreader. don't try to get an even layer of bondo, just put it on tight to the surface using the spreader in long strokes, what'll happen is the bondo will be pushed into the low areas and the spreader will sorta ride the high areas. wait for the bondo to cure, shoot primer and repeat. once you get to a point where you can sand the primer off and not have black areas, move on down through the grits to get your sanding marks out, then prime a final time