Oxalic acid would be my first choice. However, sometimes the rust is under the surface where the oxalic acid doesn't penetrate. Before I starting using oxalic acid, I came up with the following approach. First I wrapped some sandpaper around a heavy gauge wire. Strips of sandpaper could then be torn off to reveal fresh sandpaper as needed. I bent the wire a little so to avoid contacting the rest of the top tube and added extra tape around the guide for added protection.