Just do a complete WD-40 clean, (or better yet citrus degreaser) let it dry over night, and re lube. I use a hard bristle toothbrush for rust. I replace my chain after 10,000 miles now. Why? because the chain wears with the cassette and chain rings. Replacing any one of these changes everything. If I change one thing, I have to change it all. So I run it until its dead, or until I want 10 speed