i built my equipment trailer about 12 years ago. it was all new steel, but even still there was rust from outdoor storage at the steel yard. i wire wheeled it and then wiped it down with cheap paint thinner. paint was a brush coat of clean metal primer and a topcoat of rustoleum sunburst yellow - two coats on the very top surfaces. it has held up perfectly and the only rust showing is where i've scraped the paint off. it's been outdoors the entire life. certainly not as pretty as a nice sprayed finish, but for something that has to earn its living through function as opposed to just looking good, i'm content with it.
i actually am not impressed with powder coating on equipment that gets used. if you buy new and trade stuff in every few years it's fine, but if you have it long term around here it actually gets far rustier than paint. the powder coating is so tough that it remains intact when moisture gets behind it through a scratch or improper metal preparation. the rust then spreads and grows behind the powder coating, rather than just come to the surface in one spot as it often does with paint.