Well, I'm going to wade in on this subject. The absolute best primer
for metal whether it's wrought iron or any other type of metal is
Sherwin Williams etching primer. It can only be bought at a Sherwin
Williams automotive paint store and it ain't cheap. But it does the
absolute best job. I've used many different primers in the past was
repainting that handrail again in less than 18 months. And yes, it
was properly prepared as in rust removal, sanding, and cleaning,
etc.

I gotta agree that RustOleum is great and so is Red Devil, a paint
made by S.W. It seems to be as scarce as hen's teeth now, but Red
Devil with polyurethane is the best. No rust or bubbled up or
chipping paint.