Dear Good Dr., Barb, and mid-60s Buick caregivers,
Barb, I'm afraid Dusty is truly rusty. That is a common rust area for the 65. The only lasting cure for rust through is removal. The affected area needs to be removed and a new part needs to be fabricated to fit the area.
Thanks Dr. for your diagnosis. Unfortunately, I don't know things like the common rust spots for these cars. My trusty wagon has been more lucky!
Barb, you might start trying to locate a classic car friendly body shop that might be able to make this sort of repair for you. Perhaps you can find a local classic car club that can make some recommendations.
Best of luck on keeping Dusty healthy!
Cheers, Edouard