I just take some heavy grease and apply with a one inch paint brush. Even though you do not get complete coverage the oil in it creeps. It reduces the continuing rate of rusting quite substantially. At least in my experience.

Eventually it also picks up road dust. In fact a drive down a dusty unpaved road after application can stiffen it up even more. Do all of the lines though just not the areas that show rust. The oil creeps into any line mounting components where moisture gets in.

Light surface rust is different than heavy well advanced rust. How it works is simple. It blocks oxygen getting at the metal. Or slows the process down so much. You usually can just about forget those lines.