The growth of a volcano of rust like this is very bad and much harder to repair.  The labor involved is greater(more cost), and there is no way to guarantee that the rust won't come back.

Doesn't this rust look like it's really out of control ?  It's still spreading under the paint job!   These pictures are enough to give you Chicken Skin, and you can feel your wallet getting lighter.

Other Recommendations:  Replace the roof or sell the automobile.