Before you go and rip the rear wing off, if you've the time you could try the following:
1. Strip the bumper, rear light and gas filler off the car.
2. Rub down the offending area until bare metal.(will probably still be pitted).
3. Apply a coat or 3 of kurust or similar and leave it to do its thing.
4. Lightly rub down what's left until reasonably smooth.
5. Mix up some 2 part epoxy. I'd use West Systems 105 resin/205 hardener, with 406 colloidal silica to thicken slightly.
6. Spread this thinly and evenly into the area around the gas filler hole. Use a bit of pressure to ensure that all the pitting in the metal is filled.
7. Once fully cured, rub down until smooth/blended with surrounding area.
8. If needed, a light skim of filler, then primer.
9. Pop it down to a known body shop for the top coat/s.
That should see the rust held at bay for the life of the car. The idea is that the metal can't oxidise without oxygen, so the epoxy mix should seal the bare metal beneath the paint, keeping the air off. Also probably best to do the inside at the same time, (although as hidden, finish can be a bit rougher, just don't tell the perfectionists

).
HTH, Al.
