Well it's a real political style debate that has no concrete answer.
Why I'm an Epoxy First Member:
I have pic's of rust growing under filler over metal on my truck from when I stripped it.
I use only epoxy primer for my build and sealer coats and it is EASY to sand wet or dry. And no need for a guide coat because the sheen/gloss does the job instead.
A panel sealed in two (or even better 3) coats of epoxy can sit for years and then just need some 80-180 grit sanding to cut the film and your good to go with your filler or to start blocking.No need to re-appply epoxy. A sealer coat is good to tie your filler/build coats to your top coats since most filler and build products are polyester based and absorb moisture if left exposed and it will also improve your top coat adhesion. Not really needed if you use epoxy for the fill cause its WATERPROOF.
Epoxy affords two types of adhesion. Mechanical and chemical.
If you stay within the re-coat time (5 days for SPI)you get both. If not you just get mechanical which is all your ever gonna really need. (Fillers don't chemically bond to metal either, which is why you need the grit.)
Etch primers have an acid in them to try and help but they don't play well with some of the major epoxy primers... So I just say, No thank you when they are mentioned.
