Alot of your newer high end fillers are polyester based now as opposed to the old plastics they used to use, and can be put over painted or primed surfaces if scuffed up with a rougher paper. I use 80 grit. There are still plastic based fillers
out there that DO require being put on bare metal. PPG makes a good epoxy primer (CRE) that doesn't require any etch priming first. I have used this on many resto's for customer over the last 20 years with excellent results. A problem many novice bodyman have is putting too much hardner in there mud mix. The two parts together create heat and lots of it, enough to burn the skin. When mixed too hot (too much hardner) and put on bare metal that is at a lower temp than the mix, condensation starts as the filler dries. Condensation is moisture that is now trapped between your filler and bare metal. Rust is starting before you even get your fill blocked out. This is where you see Fillers "popping" out with rust under them. A job that looks great after it's done can be falling apart within months of completion. If your going to spend the money for "quality" products, then work your metal as close to perfect as you can get it without fillers, then strip it down and put a good couple coats of epoxy on it. Remember, the correct metal files can save alot of fill in the end. There excellent for removing small flaws and slight high spots caused from bumping your sheet metal. My shop has used this process for years on some very high end cars that get driven regularly and still look like the just left the shop yesterday.
You could endup with a thousand different opinions on this. The best thing to do is buy products that fit your skill level and budgit, Research them and use them like the Spec Sheets say. Never just go by what the guy at the counter tells you.
And like Etekrestos said. Avoid touching any bare metal with your hands if you can, it can and will cause unneeded problems.
