Now it is time to prepare the cores. Start by lightly sanding the cores. I use 320 grit on a long bar sander. Be careful. Lightly sand then blow off or tack off. Some of the melted foam, caused when the core is cut, will ball up and gouge the core under the sanding bar. It is nice to have a small air compressor around. I use about 15lbs of pressure to blow the foam clean. Any more pressure will blow the beads from the wing and you'll have a hole. You will notice the imperfections from cutting. Keep sanding just until the imperfections disappear. DON'T SAND TOO MUCH. You may change the airfoil.