When I did mine about 5 or 6 years ago, I replaced the rotted out areas with sheet metal that I shaped and then welded to good metal. There were more areas that were rusted, but not rotted out. After I finished with the repairs, I painted that whole area with por 15. It still looks good. I don't know about attaching with seam sealer. If you're not welding it and its not a structural area, you may be able to clean (remove the rust and rustoleum) it and use a panel bond. The funny thing is, after I went through all the trouble of fabricating the metal in that area, I acquired a parts car that was in better shape than my car in that area. I don't know if they still have it, but the car scrap metal place that I took it to is across SR-60.