OSPHO (what it's called at Marine Supply shops), if used correctly, will "convert" the rust to a sturdier compound though a chemical reaction. When the "red" stops showing through dried application, and additional coat applied, there should be no rust in the area again, especially, if finished with paint.

__________________
1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002