We live in the northern part of Michigan and during the winter face a lot of salt on the roads. That causes a big problem with rust. I've found a product called "Rustoleum Rust Reformer" that works really well to "convert rust" and provide a protective coating once dry. It's a "water base" liquid that brushes on the rust spots and when it dries, it's black (and blends very well).
I've used it on all of my farm implements, tractor, lawn power equipment, pole barn steel and the frame on my vehicles for a number of years. It stops rust from forming. All I do is use a brass brush (or steel toothbrush for small areas) to knock the rust off, wipe it with acetone and apply the Rustoleum with an acid brush. It goes on like "paint stripper" in consistency, and once dry, turns black. So far, in about 15 years of use, I've never had rust form where I've applied it. Of course, rust will form "right next to it" if the paint is damaged in the future, so I do a "quick check" of everything in the fall, just to get ready for the next "rust season".....
An 8 ounce bottle has lasted me 2 or 3 years and I use it on everything I've got that isn't "showroom appearance required"..... Even in Louisiana, it would stop rust on boat trailers used in salt water. Quite a "handy product".
Here's one link and a picture:
http://www.truevalue.com//catalog/pr...FQuraQodVwgEyA