I have a set of 50mm Carbon Clinchers that I’ve ridden for about 200 miles. I was cleaning the bike and wheels yesterday, and I noticed that there is some brake pad residue on the braking surface of the wheels. If I scrape it with my fingernail I can get it to come off, but what is the proper way of cleaning it? Can I use some sort of abrasive pad or what? I obviously don’t want to damage the surface. Any suggestions?