Paint roller covers are used with paint roller frames to apply an even layer of paint onto a surface. They are ideal for painting large areas, such as walls and ceilings. Roller covers have a porous surface that holds more paint than a brush so they can cover a surface area more quickly. Use roller covers with a longer nap, or pile height, for rough surfaces and a shorter nap for smooth surfaces. Natural-fiber covers are suitable for oil-based paints while synthetic covers are best for water-based paints.