Rust or Not?

I like Ospho and have used it a lot. Cars come and go here. 4-5 years ago I did a car and did my usual fabbing, cutting rusted panels. On the bottom part of the doors were a few minute pin holes, I DA sanded it to bare metal, rubbed on some ospho to take care of any amounts of rust that might be there, treated the inside of the doors as well with Ospho after I wire brushed it best I could. I treated the inside of the doors to some epoxy primer, and then heavy enamel over that. It took 5 years for the pinhole rust to bubble now. I feel it reason being water and debris gets into the insides of the doors trapping moisture.
Only thing else I could have done, rather than just cut it out, was to spray undercoat into the insides of the doors.
I cut that little area out other week and welded in good metal.