It works just fine either way.
A lot of the resto shops epoxy prime before fillers,
either way it'll outlast the car.
the theory is that the metal won't rust if any moisture gets
past the filler, but the truth is, if any moisture gets past the
filler, you have a problem that needs to have the filler replaced anyway.
So really it's a moot point, but yes, it's an advantage for a panel that needs
to be worked on later. Just scuff the epoxy real good before filling.
It holds just fine.