Two rules to follow if you want...
Rule One:

Whatever the brand of body filler... READ THE CAN.
If the manufacturer intended the filler to go over metal
it will say that... "apply over bare steel, aluminum" ON THE CAN!!!

Example. Evercoat Z-GRIP says on the can "over bare steel"
Evercoat RAGE GOLD does not say that!

FWIW Rage GOLD costs twice as much as ZGRIP!!!!!
but both are etching fillers..

The rule is, you can put it over bare metal IF that is written on
the CAN!!!!!! never mind what anyone else says...

If it is not written on the can,, you can do it
but your on your own...

You'll never go wrong with this approach


RULE TWO:

You'll never go wrong by first applying two coats of epoxy primer
to the bare panel first, wait a day, scuff with 80 grit and then do your filler work...

Also, FWIW:
An etch(wash) primer and an epoxy primer are TWO different things.
To the best of my recollection,
Etch primers are NOT intended to be used directly under body filler.
I could be wrong here, I don't use etch(wash)primers..
but others on here do and swear by them---
they can chime in if they want.