I used to use a product on various things and I'll provide a link. it used to be called something else. basically it bonds with the rust and converts it chemically so that the rusting stops. when it dries it turns black. then you can paint it. 1st you have to clean the rust the best you can. the product comes in sprays and jars/bottles so you can use a small model brush to apply discreetly. not sure if it would be good for your purpose but if you can clean the rust and smooth the guide surface then maybe to can apply a tiny bit of this stuff and after it dries apply cable lube.

Rust-Oleum Rust Reformer
http://www.amazon.com/s/ref=nb_sb_no...+Rust+Reformer