Por 15, rust oleum, tractor implement paint, what to use on rusted metal

My new project car will be receiving some patch panels and while some of the internal supports are exposed I would like to neutralize and stop the rust that is present. Any ideas on what to use? I have tried Por 15 in the past and followed the directions to a T and rust returned in some areas and peeled away in others. Also want to paint my floor boards and trunk too.