I may have misexplained this one. Marc describes methods to rough shape the leading edge using accuratley formed tangental facets to set up guides for the final smoothing from sanding that takes off the peaks without altering the desired smooth and properley radiused shape that is wanted to form the nose of the airfoil. It's worth the trouble as the first 1/4 of any airfoil is the most critical towards ensuring that the air flows as desired. Not that the other 3/4's isn't important but that first bit sets up the airflow for the rest. If it isn't right then you have no hope at all for the designed performace.