Body filler???

You need to get it to bare metal, and put a good coat of epoxy primer on it before you begin filling the little holes. The seams at the top skin/quarter panel should be cleaned of all of the old filler that the factory put on, and that seam made completely rust free before adding filler back. Factory vinyl top cars got plastic filler instead of lead, like slick top cars got. In those seams, either use All metal brand filler, or Tiger hair fiberglass filler. Both of those are waterproof, and will keep those seams from swelling back in a few years. Do all of your filler work AFTER you spray the epoxy primer on the bare metal and in the seams. Let the epoxy set up for a day, and then use your high build on top of that to fill the pits. Don't use spray bombs on it. Go to an automotive paint store and get some paint that will last. It's a lot of work, but it will be worth it in the long run!