Rust or Not?

It's difficult to tell until you get the paint off. Could potentially be something you could fix with a mig welder without needing to replace the metal but chances are you'll need the metal replaced there. I wouldn't have the entire quarter replaced. Just a small patch panel welded in there. It's been a few years since the repaint so the odds of there being more rust around that area is fairly low since there's no other bubbling. Plus doing it that way would make the repainting of the area a lot cheaper.

The body shop actually introduced rust on the lower quarter of my Duster when they painted it. Never had any rust on the quarter from 1971 through 2010. They painted the car and a few months later there's bubbling on my lower quarter. I took it back in and they "fixed" it and repainted it. By "fixed it", I mean they just sanded it flat, lathered some body filler in there and spot painted it so it lasted just long enough to where it didn't start bubbling again until they went out of business a few months later.