I just went thru a little rust abatement on one of my baby's, a car that was driven in Canada during the winter time by the previous owner..
The right rear quarter was rusting at about the 9 o'clock position. To get to the rust I had to dig out about two 6 inch sections of the butyl caulk in the wheel well and the boot . There was a rust hole about 1/2" by 1" on the inner fender where it meets the quarter. After a good wirebrushing I applied POR 15.
Stop Rust with POR-15® - We Know What Permanent Means!
The other 944's I have are rust free and stayed south of the mason dixon line.