We do a lot of pollishing on our custom Kenworth show trucks . YOu need the right pollishing compound and a good buffer with a couple different pads . And LOTS OF WORK !!! It will be perfect if you do it right .

The easy way would be to drive over to a big truck stop that has a truck wash and does detailing and poillishing .Show them the trailer and ask them about doing it . You might be surprised at how cheap some will do it . They usualy hit it with a strong acid first and burn it good .Then it pollishes easier . 2 years ago we had a 48' flat bed trailer fully mirror polished for a show .They did the frame rails , deck and everything in a day ! I forget the cost but it was better then me doing it . Also after it is done once ,it is easier to keep up with .;)

I owned a fleet of Kenworth trucks with aluminum dump bodies, wheels, fuel tanks, etc. I used a product from Valco which did a fantastic job. As Boot says in his post, you should have it acid washed first. Valco makes Aluminum Brightener which is their brand of acid. You can spray this on with a garden type pump sprayer (2-3 gallon size) then pressure wash it off. Then apply their Metal Polish. The acid wash is easy. The polishing takes a lot of work.
http://www.valcocinconsumerprod.com/Magic%20Mix.htm