rust!!

I am in the process of tearing down my GT and I was happy because it seemed like a really solid body. I was taking apart the back where the spare tire goes and took all that out and was still impressed on how there was absolutly no rust or anything on the tank or anywhere. But when I to the rear trim piece i noticed that on the bottom edge where the rear panel meets the rear quarter there is rust. I cannot see it from the outside but it does not just look like surface rust. so i was wondering how most of you would go about fixing this. I really dont feel like taking that rear panel off besause i know how much a pain spot welds are.