Carl, the facts are not too hard to decipher, the hard part is wading thru all the opinions.
Bob is 100% correct, yet not everyone does it that way, and they aren't "wrong" either. I think without a doubt he is right. If you want to do the ultimate, apply etch primer then your filler. Then, after filler is done a urethane primer is applied so you can final block and "surface" it to perfection.
Depending on where you are as far as climate, epoxy primer may not be that much better on that bare metal. As Bob mentioned, he visits over 50 shops and none of them use etch or epoxy. Not so much that they have some great products to "replace" one of these two, it is because they have products that "may" not need it. In other words, in your typical shop you just don't have the time to apply epoxy before filler (give me a break) and in the paint dept., most do not use etch primer for the same reason, or not wanting to spend the extra money on the etch. The products they use "get by" without the etch.
I have not seen the warrantees that are offered by the big boys in a few years, so something might have changed. But in every single life time warrantee I have ever seen, etch was included.
Personally, I live in climate where the epoxy just isn't needed. I know of not one single person who uses it before filler, restoration shop or what ever. That is NOT saying it isn't the best way to do it.
I can say this, the more I read about it, the more I feel, more people should be doing it. I think back to when I was paint repping and would visit shops right on the coast of California, epoxy would have been a darn good idea for them!
So when you come down to it, the facts are, you can apply filler over bare metal then etch (if your tech sheet recommends it, not all etches do) then a urethane primer for surfacing over the entire panel (usually over the whole panel on a resoration) then sand and paint.
OR epoxy over bare metal (or even etch then epoxy, again, read the tech sheets) then body filler, then a urethane over that for surfacing.
Choose one and get to work.
If you want to use a self etch below plastic filler you may be able to put a couple of coats of epoxy primer over the self etch to act as a solvent barrier then apply the plastic filler. Or do as most and do the filler work first directly on the bare steel then apply the etch primer. 
