It is a natural process for copper alloys. The oxidation process always takes a different time and depends on many factors. The fingerprints on the brass rim in the heat (hot room) may go dark in several weeks. In some other cases the brass rim may go dark in several months.
Unfortunately, there is no safe way to make it look shiny and get rid of the marks. Don`t worry; it is not rusting, and the prints have no impact on the sound or term of RAV`s use. We recommend polishing with a dry felt cloth. If it doesn`t help you can wipe the brass rim with 240 grit, kindly avoid the RAV`s blue coat. The rope braid protects the brass rim from touch, so the new traces of oxidation should not appear.