Listen to the answers old son. Steel goes rusty and you can't stop that. It doesn't affect brake performance at all altough it doesn't look good. My 1200 lives outside and gets light surface rust from time to time but I don't worry.
If they fit new discs, which I doubt they will do, the new ones will rust as well so it's probably something you will have to live with. Get ready for your disc and brake bolts going the same way