SunMight 2-3/4"x75' Film Back Alum Oxide Roll, 400 Grit Paper is porous causing the grains to be uneven across the surface. Film is plastic which is flat so the grains are even across the surface; this creates a more uniform sanding surface. Film Abrasive Technology is durable and tear resistant. Another advantage of Film is because the backing is plastic you can use it wet or dry. Our product uses a resin over resin bond which prevents mineral shelling and dulling