I've recently sold my BMW but it needs tidying up before the owner (friend of mine) collects it in September. They are 18" CSL reps with stubborn brake dust on the fronts. Whats the best stuff to remove it? I've got AutoGlym Alloy Cleaner which does not removed the stains. Any other ideas? I don't want to damage the rim laquer so not sure what to use. Thanks.