I have a few military pilot friends who would like to have their SPH-5 flight helmets engraved. The shell of the helmets is made from GRAPHLON, a blend of GRAPHITE and BALLISTIC NYLON. The surface is smooth and olive green and looks like it would engrave well. These helmets are rather expensive (and Govt. Property!) so I would rather not learn by trial and error. Has anyone worked with this material before?