I'm with Troy, I have never applied filler over epoxy primer in 25 years of doing this stuff for a living. Now, I must admit, it makes sense to apply epoxy first, it really does. On my next restoration project I may just do it. But I never had and I have NEVER, EVER had a failure (ok, that I know of) with applying filler over bare metal. It is after all THE way recommended in EVERY tech sheet I have ever seen for plastic filler.
Understand, I agree with the epoxy over bare metal idea. Many EPOXY manufactures recommend applying plastic filler over the epoxy. HOWEVER, if anyone can come up with a PLASTIC FILLER manufacture who has in the tech sheets to apply thier product over epoxy please post it here.
All I am saying is applying plastic filler over bare metal "may not" be the best way, but it is the only way according to the manufacture of the product and that can't be "bad".
As far as "most pros" doing it over epoxy. Honestly, I have been in hundreds if not thousands of shops and I can not remember a single one of them doing it, even restoration shops. I am in sunny California, maybe that makes a difference. But I don't remember seeing a single one.