I bought a used utility trailer that needs some fresh paint. I've thought about plasti-dip, but will it hold up for this application?

If not, what should I use as a decent paint that I can apply (Roll or spray) at home?

I would say a couple coats of POR-15 with a roller.

If you have access to a sprayer the best thing to do would be to go to Tractor supply and buy some cheap paint and go to town on it.

POR-15 is a great product, too. Its kind of pricey for a utility trailer.

I was just offering a longevity solution. Then again, it is just a utility trailer. Probably wouldn't even bother with it honestly

POR15 is expensive stuff iirc. I wouldn't waste it on a trailer. I would say how you go about it depends on the tools you have.

How I would do it:
Pressure wash as much of the dirt, crap , and rust off as is possible.
Go over it with a wire wheel as fast as I can if the rust is heavy.
Spray ospho all over.
Next day spray second coat of ospho if the rust is heavy.
Heavy coat of red oxide primer. 2 coats of rustoleum on whatever color you like. I prefer dark gray.

If you don't have an air compressor and a spray gun, now is a good chance to buy one.

POR is like $150 a gallon on amazon. So yeah, kinda expensive. But, i think one gallon would come close to covering (i think coverage is 350 sq ft).

Original color looks like white/purple (WTF?). Its gotta be painted.

I just used a bunch of spray cans of rustoleum on my trailer last year when I replaced all the wood and repainted it. Did wire wheel the hell out of it first.

POR 15 by itself will not hold up when exposed to UV rays. It has to be topcoated or it will turn chalky in no time.