A sanding block is the foundation of flat, even sanding. Without a block, you unconsciously follow the contours of your fingers and hand, creating waves and high and low spots in the surface. A hard block delivers a crisp flat result; a soft block follows gentle curves without damaging them.

Interface pads are soft intermediate layers between the sanding block or backing plate and the sandpaper. They provide more flexibility on curved surfaces and absorb minor irregularities. Without an interface pad, you're sanding with a rigid disc directly on paint, which quickly leads to uneven spots.