One rule of thumb is anytime you apply ANY product over another product, the product you just applied is dependent on the product under it for adhesion. If the first product applied fails everything on top of it fails. When I was a body man epoxy primer wasn't invented yet so all we had was primer/surfacer (similar to what is called "high build" primer today). It was adequate for the time but epoxy primer is much better at both adhesion and metal protection. I've read you could coat a panel with epoxy primer and leave it at the bottom of a swimming pool for months without it rusting. I've not tried that
but I would apply the epoxy primer first then the filling primer.
