when i helped my m8 do his, we screwed the rear spats in, filled over the holes, fibreglassed around the edges for more strength and then used filler to get the finish...for the skirts, we just installed them normally and then fibreglassed over all the edges and corners and then sanded it to get the final shape and filler to fill uneven areas to finish it....looks cool, he did the same for his rear bumper on his other car just fibreglassing over the panel lines filling it, sand down and use filler to get the final shape to blend into the body, in all the finish looked good and had not cracked or anything since then....