I've used other industrial 'no-rust' paints before... They're pretty much all the same.
In fact, just last Summer I had my 16' open car hauling trailer sand blasted, then epoxy-primed. I topped that off with a spray paint job of an industrial no-rust paint made locally. We'll see how good it holds up to life outside in Cleveland weather! They were having a sale on 'Hunter Green' paint when I bought mine, so it was a tad under $30/gal, normally $50-60 depending on color.
Actually, a farm supply store might have good paints as well... They sell it for tractors and other farm impliments that spend their entire lives outside in the weather... I was planning to paint the trailer Ford Blue (think blue Ford tractors), but the green works, too. Not too many other trailers that color around here...
28 years ago when I was actively working on my car, AlphaCat, I had tried to use Rustoleum with a flattening agent. Didn't turn out well... and it took forever, and I mean months to cure/harden. Then a few years later, restoration vendors like Eastwood started coming out with the proper 'frame paint'. Then epoxy primers... The frame paint and epoxy primers do not have the UV inhibitors for things exposed to direct sunlight. I'm just sticking with a base of etching epoxy primer followed with a light coat of good old gloss black acrylic enamel with a flattening agent to get it to the proper duller sheen that should hold up for years...