Not all paint is the same. Some are impervious. Most of the two part epxoy paints are realy tough. Water and moisture are tough and you can just look to the automotive industry and you will see manny choices but application, substrate and topcoating all go hand in hand in preventing rust or corrosion. Proper application is a must and cost is high. But you have already covered the metal and frankly your stuck, somewhat ,with what you did.
I would recomend you an epoxy grey enamel appliance paint which is not all that expensive and a quart goes a long way. It has held up extremely well for me. Outside Emron ( $8-10 a spray can) is very good and if prep'd properly and adaquate coating is applied can last a long time. Por-15 is fantastic directly over rust, but at $35 a quart you should get something for your money ( no comparison to say "Rust Oleom") but is not good from a UV point /
sunlight.
Quess what i am saying is you may have started bad, but you can refinish rather quickly since most of the labor was in prep work. I would recomend you do it now and not put crap on something that may have been inferior to begin with.
That's my nickle,
RICH