Here's my tip: Use wet fine sanding (600- to 800-grit wet sandpaper) to wet sand the smooth sides of the board (the entire underwater side as well as the sides) with plenty of water in the longitudinal direction. Then rinse well with water again. Not too deep scratches as well as colour marks from bumps can be polished away in this way. The board then looks like new again! This also works with the fin.
Reader tip from Sebastian Schütze