The Premium category features a 577F abrasive, the perfect belt for serious metalworkers. These belts use a Aluminum Zirconia abrasive on a very durable backing, and contain a grinding aid to reduce heat. The result is a longer lasting belt. Also, our Premium category features 3M's unique Trizact abrasive. These belts will provide the smoothest, most consistent finish of any coated abrasive. These belts wear incredibly evenly throughout their long life.