what kind of paint should be used to paint a old metal semi trailer box and what kind preparations are needed?

Answer 1

I know someone who did this to a horse trailer, years ago, and was very pleased with the results.

You have to remove ALL rust. She rented a sandblaster to do the job, but there are other methods. If you paint over rust, you'll just have new rust.

Next, make sure the surface is absolutely clean, with no traces of sand, dirt, cleaner, grease, oil, or anything else, including chipping, peeling, or alligatored paint. This may take a power washer.

Now make sure it gets completely dry.

Paint with a rust-inhibiting outdoor-use enamel over a primer for metal. If you find a paint specified for metal, that's even better. You want to use a sprayer (either a bunch of cans or a spray system you can fill with your own paint) rather than a brush. For best results, apply two or three very thin coats for best coverage.

Answer 2

Blast it as clean as you can, let it dry, use a rust-stopping metal primer and metal paint, and sealer. It won't stay on forever, but you should get several years from it - less if you haul the trailer on the roads.
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Answer 3

scrape off any rust, wash it, prime it that has a rust inhibitor in it, then two coats of paint, hammered paint looks great on a trailer.

Answer 4

clean well with a pressure washer, wipe down with mineral sprites and paint with a good quality primer made for aluminum and then paint