I have a white 4 place enclosed trailer that I recently did the outside after about 2 years of no waxing, basically rinse off-n-be done. Anyways there was the oxidation, stains and rust particles as I live by a foundry. It was a lot of work but it looks almost brand spanking new. Here's what I did
Washed with liquid soap and water, while still soapy I sprayed simple green on and worked it on the sides. Rinsed
Then I used Turtle Wax's Clay Bar kit. The step 1 (liquid clay) was amazing on getting off the stains and what not. I even went one step further, I removed the metal screws (if they would come out) hold the sides on. So that I could get the stains around the screws also. I did one row going down at a time. Then once done, I rinsed and dried.
Then I used the clay bar to go over the whole trailer.
Then I gave it 2 coats of wax. The sides are as smooth if not smoother than new. It was a lot of work but the results are awesome. My trailer is 10 years old and you would not know it now.
I should add that it took about 2 weeks to do this as had to work on it when I could find the time.