Sanding abrasives are used for general applications such as surface preparation, deburring, cleaning, finishing, and polishing. They are available in multiple forms for hand or machine use. Sanding sheets, hand pads, and sponges are primarily for hand use, while flap wheels, belts, discs, and specialty shapes such as stars, crosses, squares, and spiral bands are designed for portable or stationary rotary machines. Coated abrasives are formed by bonding an abrasive mineral to a backing. Common backing materials include fiber resin, cloth, paper, and film. Fiber resin- and cloth-backed abrasive types are more durable for heavy-duty use, while paper-backed items are more flexible and economical. Film-backed abrasives are best for fine finishing work. Waterproof backings are available in some cases. Coated abrasives can have a closed, semi-open, or open coat type. Closed coat sheets have 100% of their backing covered in abrasive grains. They are the most aggressive but produce finer finishes. A semi-open coated sheet will have about 70% grain coverage, while open coat is usually around 50% coverage. These coat types cut less aggressively but also produce a coarser finish because of the grain gaps. Non-woven abrasives are formed by dispersing abrasive grains throughout a synthetic fiber mesh. They tend to be less aggressive than coated abrasives and are commonly used for surface conditioning and finishing. When a density rating is provided, this describes the number of fibers that have been compressed into the mesh. Softer densities are more flexible, while harder density abrasives are more aggressive and longer lasting. Non-woven abrasives are always waterproof. Common abrasive materials include aluminum oxide, alumina zirconia, ceramic, silicon carbide, talc, and diamond. A wide range of grits and grades are available in each of the categories.