You should - it's actually one school of thought.
There are as many people out there who say filler first then epoxy as epoxy then filler. Pros and cons for both methods.
Not all the pros shop lay on epoxy primer before filler, many people still use the bare metal filler method.
I did say "all the pro shops I know", I'm talking 20k plus paintjobs not dailys
The way you are saying it would leave anyone with the impression it is the only correct way.
This is a job I'm hoping to get to soon, and the whole "filler before primer" argument is one I've heard a lot. The main problem area on my car is the roof, and it's an estate car so it's a big roof. It's covered in areas of pitting. My plan was to clean out the rust (mixture of acid, then spot blaster) until clean, leaving me with nice clean pitted areas. A chap I know recommended primer first, then filler, though the pain supplier said never prime first, as the filler won't stick to it, and in fact the instructions with the primer say "don't fill on top". Then above we hear about keying the primer - but how can I do that inside the pitted areas? Some of the pits are very small indeed (though they won't be when painted), I can't get paper in to key them.
I don't like the idea of filler with nothing under it, so I'm relieved that lots of people now suggest painting under it, but I can't see how I can key it. Or will the fact that it's pitted provide enough of an anchor?
To add to the confusion, the primer I have bought (but not used yet) is called Upoxy Super Etch, which implies from the name (but does not say in the data sheet) some kind of link to Epoxy. Or maybe that's just me. But in the notes it says "SUPER*ETCH Primers cannot be overcoated with body filler or polyester spraying filler. lf it is found necessary to overcoat with body stopper, cellulose or acrylic knifing stopper may be used."
I really picked that based on a conversation with the seller (who has been selling paints etc for some years, so I'm a trusting kind of person, have to get advice from somewhere) and the fact that I can brush or roll it on. I can't spray 2k stuff without killing myself (though that was disputed by another chap I know, a painter by trade, who tells me it's OK with a HVLP gun).