JS,
I've used tractor paint on quite a few vehicles and trailers and am completely satisfied with the results. Preparation is the key. Remove all rust preferably by cutting out and replacing. I used a product called "Right Stuff" for areas I wasn't able to access. For spraying the frame buy a cheap gun from Harbor Freight and spray. I've picked them up as cheap as 10 bucks. Will cost you that much with spray bombs. Have fun,listen to all the guru's and make your own choice. Pics below.

Dan
http://rides.webshots.com/photo/2750634870071274430qzyNpf