Thank you jdcrawler and canawler. I was always taught too, to prime the metal first, before applying filler. That was the reason for my question earlier. I have tried using filler on bare metal before, and it seems within two to three years later, the filler starts to raise from the metal, which is the moisture separation between the filler and bare metal.
Depending on the depth of pitting to be filled, I spray primer into an empty soup can, or spray can lid, and use a small round brush to dab paint into these pits, let cure, lightly sand, and just keep building up the pitting this way, until the paint filled pit is even with the standard surface. I find this to be the best way to adhere, and don't have to worry about separation years down the road. Yes it takes a little more time, but I haven't had any problems.
Edited by johndeereelfman, September 23, 2012 - 11:15 AM.