Here's my two cents on the filler issue. I don't believe in putting any filler over the top of a primer, whether epoxy or not. The rule of thumb to always follow is that your final project is only as strong as your weakest bond. Primers adhere excellently to filler and filler to metal, but not so much primer to metal.
Of course, I don't apply the filler directly over the top of the metal either. As EdP touched on, it's generally best to do an "etch" first. This is typically a phosphoric acid based solution and is also called "rust-mort" or "rust converter", but is NOT navel jelly! Once the metal is bare and clean I take a small amount of etch on a rag (wear gloves for pete's sake, this is acid!! ) and wipe down the surface of the metal. Rust is oxidized metal; as such as soon as you expose the metal to oxygen it will start oxidizing. Whether or not you can see it, if the metal has been exposed for ANY amount of time it will have oxidation. The etch stops the oxidation cold and keeps any futher oxidation from happening. The etch will also leave a slight film on the surface which can be removed with a quick wipe down of wax and grease remover. A gallon of good etch is about $30 at your local paint store and I feel it is an essential item for anybody doing body work.