If it is rust the culprit, you can remove it and immerse it in Evaporust. On the vehicle you could brush on naval jelly at points of rust. You have to be patient, let the chemicals do the work. Every hour or so go in and wire brush, pick at area of rust scaling, and reapply the naval jelly. Both will remove rust, and convert it to black oxide. Except that Evaporust is more of a solution so it removes more so. The naval jelly you can dilute it with a bit of distilled water to a solution, but you are decreasing its effectiveness equaling more wait time. Don't use tap water, instead distilled water is your best choice.
If it is mechanical than you check the flap for alignment and other things.
