I'm facing the same dilemma with my 81 SD.
Both fenders rusting - one on the lip near the front bumper and the other just above the trim panel just ahead of the front door.
Bought a starter kit from POR to stop and stabilize the rust then contemplate my next move. Since the car is more of a heavy user rather then a restoration project, a small patch of touchup spots won't bother me too much. Still better looking then rust spots.