sand something down
1. to make something smooth by rubbing it with sandpaper. (To act on the main body of the object, not the imperfections.) You should sand the board down before you paint it. Please sand down the board.
2. to remove bumps or imperfections on the surface of something by rubbing them with sandpaper. (To act on the imperfections, not the main body of the object.) Sand these bumps down, will you? Sand down these bumps, please.