Coated with aluminum oxide, 3M™ Softback Sanding Sponges can be used wet or dry when sanding on wood, fibreglass/gel coat, marine paints, metals and plastics. They are flexible and conformable to allow for hand sanding in areas inaccessible to machine sanders. Medium ranges from grade 120 to 180 grit. This full thickness medium sponge is perfect for preparing old bottom paint for a new coat. When used wet, it keeps the air clean from dust. 3M™ Softback Sanding Sponges are coated with aluminum oxide mineral. Each grade uses a range of mineral sizes. Medium ranges from grade 120 to 180.