I paint for a living and deal with steel all the time in the industrial plants. Sand it down with a flap disc on a angle grinder, prime with a acceptable primer based on your top coat.

Pointers- Epoxy is a great material to use but depending on color it will chalk up and fade. Generally at industiral complexes we use a epoxy primer followed by a epoxy mid coat followed by a urethane top coat.

Urethane will hold its color and last much longer in the sun than a epoxy. Epoxy protects the steel and urethane protects the epoxy.

I just repainted my trailer and used a enamel primer and then covered it with pittsburgh sil shield modified silicone enamel and it looks awesome! This is a newer product and lasts a LOT longer than a standard industrial enamel.

Here's a link to some info. My cost was somewhere in the 50 per gallon range.
http://buyat.ppg.com/rep_pafpainttools_files/porter/tdb/95-5000.pdf

You can use a standard krylon spray primer (industrial) if needed and it will do just fine. let the primer set up for atleast 24 hours and it should not wrinkle the primer.