Climbers sand their fingers to prevent skin injuries such as flappers, cracks and tears. This happens due to calluses building up.
Climbing is pretty rough on the skin, and after doing it for a while, you start developing calluses. This is a natural process of the body and is meant to protect the skin. The problem climbers have, is that after some time, the calluses can actually build up and cause potential skin injuries.
The skin injuries can occur due to different reasons, such as the callus area being very dry, or when a callus is caught or pinched on a hold, which can cause the callus to peel off, which is known as a flapper.
Climbers tend to sand down the calluses to reduce their size, and make them a little bit smoother so that they don't get caught and rip off.
