We painted our trailer about 2 years ago
I absolutely second the angle grinder. We tried sand blasting and it just was not powerful enough to remove all the rust, and it took a LONG time. Also tried a 'paint remover' - looks like a buffer but with a sander, also didn't do a very good job. In the end, our trusty angle grinder with a wire wheel did the best job.
Here's a pic of me grinding
Definitely wear gloves, eye pro, and ear protection as well- it's loud. My husband and I took turns because the vibration of the grinder kills after a while.
Another trick we used was Rust Converter.
It converts rust into this black substance, and gets rid of the rust. You still want to try to grind everything you can get to off FIRST but this stuff was awesome for getting in the nooks and crannies and ensuring all the rust got killed! Ha.
Here's the best pic I could find of the trailer after we applied the first coat of rust converter (had to use the trailer for a rodeo)
After grinding and 2 coats of rust converter we did 2 (we may have done 3?) coats of primer and 3 coats of paint. We used an air compressor and a paint gun attachment. Definitely get some masks for when your painting, a respirator would be best but we didn't use one. We were outside.
We also got some Liquid Nails and caulked that into the seams between panels on the roof to waterproof it. It's still totally waterproof (unless it comes in the giant open windows on the sides of course) but the manger is safe for tack and hay.
It's perfectly functional, has resisted rust so far (it's been 2 years), and looks well enough for us :)
The only other piece of advice I can offer is, however long you think it will take, triple it. :) Good luck with your paint job! Make sure you take lots of pictures for us! Haha