Abstract: A method for fabricating an impervious fiberglass blanket having a plural of spaced-apart slots on its inner side, such as for overlaying and venting a pressurized helicopter rotor spar, includes the steps of positioning a thin metal forming grid of spaced-apart strips, corresponding to the desired slots, on a forming table over a sacrificial layer of peelable material. Sheets of resin-impregnated fiberglass material are positioned over the grid, another sacrificial layer is applied, and a flat caul plate is placed over the lay-up. The laid-up assembly is placed in a vacuum bag, the bag is evacuated, and the assembly is cured in an autoclave. The sacrificial layers protect the blanket from mold release and outside contamination.