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Will look into it, thanks. Do you know if it builds up faster with more driving or from not enough driving?
It will build due to moisture getting on a raw metal piece.... it will build when you wash the car and not dry the rotors, it will build when it rains, it will build when there is humidity.
Take them off, wire brush, and paint or powdercoat....
Stock rotors are made out of plain,non-coated steel. The reason the main surfaces don't rust is because the they get polished every time you step on the brakes. The rest of it, however, will look just like any other piece of steel left out in the elements, over time. Yes, it does get to be unsightly. The solution, as suggested above, is to remove the rotors, sand, wire-wheel, or sandblast off the rusty places, and coat those places with a rust-resistant coating. Alternatively, you could replace them with custom rotors that are coated, or made out of a non-rusting material (stainless, ceramic).
It's iron. Iron + water = rust. There's nothing you can do to keep it from rusting unless it's always kept in a climate controlled moisture free environment. It's going to rust no matter what you do. Painting or powder coating it will be worse because it will just come off and make it look crappier than the rust. The only way that I know of to keep it from rusting is to get some rotors where the hats are aluminum instead of iron. I have 2 piece rotors that have aluminum hats and they don't rust. But they are pretty expensive.