Sand paper does not reduce friction. Think of it like this; Does a basketball bounce better on your concrete driveway, or on the grass? Basketballs bounce better on the concrete because it is a rough surface, it creates friction, which makes the ball bounce. Sandpaper, like concrete is rough. It works the same way. Another way you could think of it is like a playground ball. They bounce high, even on soft surfaces, because the treading on the outside of the ball helps it bounce, creating the friction that soft surface couldn't. The treading works the same way as sandpaper does.
