They build doors like that intentionally. They want you to buy a vehicle every 7 or 8 years. This is what I've done that's worked well for me. Pour your rust remover with zinc inside your seams if you want to go extreme dip them. You can use a piece of gutter. Let them soak for a day or 2 let it dry thoroughly and then seal it with your moisture cured aluminum pigmented primer or epoxy .In a industrial environment the moisture cured goes down 1st, 2 coats , followed by 2 coats of epoxy primer then a topcoat. This system is used for very corrosive environments. This is how I paint my stuff.