Kier,

Epoxy primer will seal the metal - a little rain will not get through to the metal (if the epoxy was properly applied).

The metal should be rust-free before applying the epoxy.

POR (Paint Over Rust) is costly over-kill when applied over epoxy. MetalReady Prep is for non-painted metal.

Preferred sequence: POR over rusted (Prepped) metal and then epoxy over the POR before it has set. The epoxy then gives a good surface for over coatings.