Best way to slow down this rusting?

I had sanded and sprayed primer the rectangular shaped area few years as seen in first photo. I am guessing I didn't do a great job? The rust didn't go past this area which leads me to believe it was worst after I sanded and primed.

At this point I'm not too concern with aesthetics to a certain degree. Just want to make sure I'm not driving a skeleton in a few years.