I agree with Mr. Delano from a few posts back. You cut out old rust, put in bondo, it gets wet and traps water. Then contact point of the bondo and metal rusts out in a ring.

A friend of mine put himself through college in the 70's doing rust repaints in Buffalo. I was the "works for beer" guy who hung around trying to learn his tricks. His technique was to cut out the rust and form a new panel of galvanized sheet metal as a patch. He would overlap the good metal by about 1-2". He would then use a brick behind the metal as a form and hammer to make a small divet or indentation in the overlap area along the seam. The rivets went in divet so they did not show. Bondo in the small overlap area / divet only to cover the seam / rivets. Roofing tar or undercoating behind it to prevent moisture from getting in. A gallon of Dulux enamel and the car looked great for a couple of years.

Chuck