Your post doesn't have what year, make, model, front, or rear so I can will give some general advise. In the location pictured the outer and inner fender/quarter are either touching each other or very close hence trapping water and dirt and causing the issue you have. If you can't get between these to address the rust between them, you are screwed. That is worst case. If you are lucky, you have a few stone chips that took out a chunk of paint and allowed water to get under the paint and start the rust process. Treating the back of the panel, and I mean the outer panel, if you can get to it will help but it is not the magic bullet. You still need to take off the loose rust and get down to something solid, thinner metal, but still solid. If you can do that, the rust stop will work fairly well. After the rust stop dries I would still spray with epoxy and a sealer even though the epoxy in that location will not see the sun. You still have to fix the exterior damage. I usually take a small pick and press fairly hard on the area you want to repair. If it pushes through, so be it, you were going to have to fix it anyway. If it holds, you might get lucky a second time. Don't use a angle grinder to work this small area, instead break out the Dremel with a stainless wire wheel. You are going to try brain surgery not ER work. If the stainless wheel can get you to good metal with out making a hole, you are three times lucky and should play the loto. The metal does not have to be perfect, just clean. Small pin prick size dark pockets are what the rust stop is made to address. Spray and allow the rust stop to dry a few days then sand off the excess. You want your glaze to stick to the good metal around the little pin picks (if there are any). I like 3M Platinum because it can be used on bare metal. Sand and glaze again, and again, until you have it close to the original metal. Prime, paint, and feather as needed. If you punch a hole, no worries, grab a piece of copper and use it as a backer and then weld up the hole. This will burn some paint and make the repaired are much bigger but it will fill the hole. Hope this helps and good luck.
