[lupo]

Good rust work starts from the inside out. This area is going to have to be opened up, cutout, and patch installed. Open It up the you can clean it up with a phosphoric acid solution and follow it with good anti-corrosive sealer. On your next trip to Harbor Freight pick up a couple of engine cleaning guns in the air tool section. Have used these things for years to spray rust remover / and sealers in hard-to-reach areas. U punch a hole ,put the nozzle against the hole and give it a quick blast. This engine cleaning gun will blow paint and rust remover about 3 ft and it's great for rockers and windshield posts. Good luck with your project.