Here are a few products for consideration:
I’ve personally never heard of Rust Mort, but a quick search show it to be very popular. In checking a few other sites some seem to think it’s the best product on the market while others prefer POR Prep & Ready because it doesn’t dry sticky:
http://www.por15.com/PREP-READY/productinfo/MRG/
And if you believe the propaganda from Picklex 20, it’s the solution to every problem out there:
http://www.picklex20.com/1142290.html
My own experience with Picklex 20 is that it will preserve bare metal indefinitely as long as it is kept indoors and dry. About 10 years ago I had a mint condition GMC grille soda blasted to bare metal. I sprayed it with Picklex 20 and it looks exactly the same today. I’ve also used it over bare metal when sitting around in the garage for weeks or months and it keeps the metal completely free of rust. I just can’t think of a specific project where I’ve deliberately used it as a prep agent under paint.
I’ve used both of the above products along with Corroless, but haven’t used any of them inside a door.
I was somewhat disappointed with previous applications of POR-15, so when it came to the doors on my 1960 GMC I stripped everything bare that I could and coated everything with Zero Rust.
http://www.autobodystore.com/new_page_1.htm
http://zero-rust.com/
That was done in 1998 and the inside of the doors look exactly the same with no deterioration.
Once the ZR cured I applied 3M Body Schutz as a sound deadener/top coat. If I had it to do over again I would have used U-POL Raptor bedliner inside the doors instead of Body Schutz because it’s a hardened product.
Good luck with your project.