In order to stop panel rust in specific areas, especially in the quarter panel where it meets the bumper, there is some access from behind that you can't get to unless you literally cut that part out of the car, so sanding it and repainting it with a rust inhibitor isn't going to do anything, it will come right back from behind (don't ask how I know!) but if you can sand down the affected area and also get behind it (such as a fender) you can stop the rust effectively by applying the rust inhibitor on both sides.