Actually rust is not much different than cancer
in a human being, once You get it, it more than likey will be back...but it sounds like You chose the best option, excluding buying different doors..
Primer is not waterproof, but paint is, so I
would spray paint on the inside of the door where You made the repair...it will help You buy time with
the rust problem....BB

1952 Chevy 3104 Shortbed 5-window
Remember; Politicians and diapers both need to be changed
often, and for the same reason.....