I had that same rust on mine. I grinded and sanded it down to bare metal, sprayed it with that rust neutralizer stuff, and primered over it. It still came back within a couple months. So now I have to re-do that whole process every couple of months. If you have the money, go to a body shop, have them totally cut out the affected area and weld in new sheet metal. Thats the only way to really take care of the problem, and not just prolong the slow decay process. I like that cancer analogy, thats just about right.