3M Fiber disc 381C by adhering aluminum oxide mineral to a fiber backing. The closed coat construction on 3M Fiber disc 381C means mineral grains cover nearly 100% of the backing, so the abrasive is durable under pressure. The stiff fiber backing is extremely durable, withstanding wear, while standing up to tough applications like beveling. - 3M Fiber disc 381C is a versatile and general purpose metalworking disc, designed to help with a variety of applications from grinding, weld removal and finishing to blending on common metals like carbon steel.