The problem with rust on doors is that it usually starts from the inside where water and debris collects in the front bottom corner of the door skin. The best way is to cut it out and weld in a patch. Otherwise, you need to really clean out the inside of the door (sand, leaves, loose rust) and apply a rust convertor made by loctite or rustoleum, then apply a protective coating on top (POR-15 or some paint), but this will only buy you some time; it's not a permanent repair.