the only stuff that will treat the metal and prevent corrosion adequately is all extremely chemically reactive(requires industrial handling) or expensive (laquer paint, powdercoat).

you could do an industrial metal finish in an acid bath or plating process, but that's going to be on par with powdercoat pricing. your only cheaper alternative is an enamel spray paint but you're going to be at at least half the cost of a powdercoat to do that right and the finish isn't going to last as long, be as good looking, or have the variety in color choices.

if you want a raw metal frame, take it to a metal finishing place and ask them if they can strip it, acid bathe it, and clear coat it. if they can't do that for less than you are willing to spend, sell the frame and buy a raw kilo.

in fact, your frame sucks so much it's not worth doing anything to; just sell it and buy a better frame.