If they were my doors I would just drill a hole in the bottom of the doors shake or vacuum the rust flakes out then get a hand pump sprayer like walmart sells and use rust converter its real thin and sprays nicely. You should be able to soak the inside of the doors and with a hole on the bottom it might make a nice drain if your window seals ever leak.
I never used POR 15 if its thin im sure it would work great or use something like this
http://www.gemplers.com/a/shop/produ...&GEN9=0&T1=RCQ