I was just stripping the step sides on mine today. I'd sprayed the under side of these just a couple years ago with rustoleum and as I was taking it off I thought to myself "This is why painters don't use it" .
If your going to use an epoxy primer it needs to be against good clean metal not over rattle can primer.
Thats like using wax paper under a band aid on a cut.

If your just doing a panel at a time just use a small 6" roller and go with 3 good coats epoxy from the get go then if your years into the project you know its still water tight in epoxy instead of wondering about how much moisture the rustoleum is holding.

Give me ambiguity
or give me something else