I always thought you wanted to apply fillers to bare metal for maximun adhesion so I think you are ok Lou, but I am also no expert. That's how I did it on my car anyway.
Todays fillers/glazes can adhere to most all sanded substraights meaning paints, primers, sealers and bare metal. I among many would advise NOT to go to bare metal unless it is absolute! When grinding to bare metal you expose the steel to moisture.
Most folks use parts store special "Bondo". The absolute worst thing you can waste money on. Most products of this nature need Physical adhesion i.e. grind/sand scratches. Crap shrinks and cracks and eventually pops off after rust has covered the bare steel that was exposed before the filler was applied. By the way, fiberglass needs the same "Physical" adhesion on steel. Don't use it either imo..
S.E.M. and Evercoat have the leading fillers/glazes available in my professional opinion. Kent also has some good products.
