The best stuff I have found for managing rust is Ospho. It converts iron oxide [rust] to iron phosphate and stops rust. It is thin / watery and can be sprayed with a plastic spray bottle, squirted, brushed, or rolled on. I learned about it while living on and maintaining my sail boat. Ospho did a good job of treating rusted steel exposed to salt spray and salt water. Leaves a white powdery surface that can be washed off and painted or left as is.
I've used it on my 2000 D2 that started out as a fleet vehicle in Hawaii before I bought it. The treatments completely arrested all the "Hawaiian cancer" that shows up after a few years in Hawaii. No problems since treatment even with road salt in Colorado winter roads.
If the surface has been previously treated with waxes or oils, I wouldn't bet on it penetrating and working as well.
