That's actually how the rust started on mine...

and it just went crazy after that..in no time I had the huge spot you see in the picture.

My stepson did grind it down to bare metal... filled and painted it.... with in weeks..it ate its way right back thru the filler/paint..... So obviously the only real way to fix it... is to replace that area with fresh metal. Which is what I had done to mine recently over the winter...

Maybe...
My mistake when doing the repair with grinding,filler and paint...
May have been that I didn't use any kind of rust killer....

You could try grinding it down to bare metal... Then treat it with some pors 15...It suppose to stop rust dead in its tracks! we didn't do or think to do that... Maybe if we had... It may have not returned for a while? or not at all?.... One member on here went as far as replacing the whole quarter panel but he let his get way out of hand. But he did do it him self and it turned out pretty good.

The problem occurs from the little hole in the area of the fuel filler behind the door. the water runs down that drain hole and behind the quarter panel... Theres also some foam type material there that holds the water thus causeing the metal to rust ... Thus getting that typical 5th Gen rust spot. The bump shop ripped all that crap out when they repaired mine....

What part of Michigan are you from?