Incremental body and paint advise

If you use something like Ospho be sure to apply a second wet coat later, let it set for 10 minutes or less based on temp. and wash good with water. That neutralizes it. Different primers and enamels can react to that acid treated metal although it is totally dry. Sometimes months later. Be warned.
Reason why to use an epoxy primer over the bare metal is epoxy sticks like crazy. and seals. Body filler tends to absorb moisture if given the chance. If you have bare metal, would you want a damp sponge on it?
Different co.'s have different primers with different chemistry. Each can react to whatever conditions differently.
Not saying I am right or anyone else is wrong.