I have repainted and touched up my trailers several times.
I would just clean & lightly sand all the angle iron, square or round tubes, and any other larger surface areas. You really cannot sand effectively any of the expanded metal surfaces without a sand blaster but that ok, just clean the surfaces of dirt and grime(maybe a power washer but not necessary). Then I would use semi-gloss enamel spray paint cans or borrow a sprayer and have at it. Ideally you would want to sandblast that trailer for a perfect prep job before painting, but this is not a porsche with a perfect paint job and clear coat. I have done this several times and the results are very good.
Also, I assume you are taking about an open trailer.