Each skin half has eight stiffeners that will ultimately be riveted to it. These stiffeners, all 16 of them, have to be cut, trimmed, deburred and dimpled. I used the aluminum shears to trim them. The grinder with scotch-brite wheel, really paid for itself by making rounding of the stiffener corners a snap. The stiffener holes were pre-drilled which was nice. It took me three nights to do all 16 stiffeners.