Yes, that's what I was taught in a tech school class I recently took. You actually seal the bare metal with a reduced coat of epoxy. You can then use body filler on top of the epoxy primer.
In my years of painting, its better to do any body work on bare metal,then etch-prime for corrosion purposes,and then epoxy prime. At least thats how we learned at Wyo-Tech years ago.
good replys in the day we used red oxide etching primer.. Ford emmm had the best stuff, then when finished use a filler primer etc to finish. Then depends on paint use the primer for that paint...
I just went through this and went with epoxy primer as recommended by a friend who is a professional bodyman. If you look in this forum I think that is the consensus too. http://autobodystore.com/forum/index.php