I agree with these guys. Blasting and sealing right away is your best bet. I personally don't like to metal prep the area since it leaves residue down in the pit forming salts that you can see with a magnifying glass and these will blister your nice new paint. We found this out the hard way when I was doing high end Model "A" Fords. The fuel tank is part of the cowl and dash assy and they receive full paint and the tops of the tank around the fuel cap are usually really rusted. Of course in the AACA judging criteria the cars must be all laquer but we did use epoxy primer under coats. We never had anymore problems with rust pit blisters.Good Luck.