After stripping and prepping the inside of the roof with POR15 I discovered the roof had a section of oil canning going on. The POR15 is under the gray tie coat. I cut some 1/8" x 1" strips of flat strap and shaped them to the roof curve. I then set them in seam sealer and pop riveted them into the structure being sure to miss any headliner bows mounting points. This totally stopped any flex that was going on. This is also something I had with another Challenger where the roof skin flexed so much it made sanding the roof a real pain the metal gave under the slightest sanding pressure.