Soap stops the sanding grit clogging up with metal. Aluminum galls into files and sandpaper more than any other common metal. Sandpaper ELI5: Sandpaper is rated by how many 'grits' are in each square inch. 80 grit is rough with 80 large 'grits' per square inch, 800 has 800 small 'grits' per square inch.