The Cycle Clinic abrasive block is designed to clean braking surface of the rim sidewalls or disc braking rotors. This product is made to rediscover optimum braking performance. The block can be used on a dry or wet surface, with soapy water or not, as a very fine sand paper. For better result use firstly rougher CC N20 for basic clean and then soft CC N40 for the final finishing to get excelent performance.