I've seen this on other mercedes as well. apparenly the fuel is leaking from the cap somehow falling on the paint and ruining it. i'm attaching a picture.
is there a soltuion to the ongoing problem, and i'd appreciate any advice on how to fix the paint that has been faded.

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Whether you think you can or cannot, Either way you're right!. by Henry Ford.

My suggestions:

1. Buy a new gas cap.
2. Check to make sure the overflow tube is not plugged.
3. In the event of a fuel spill, rinse with water immediately.
4. Keep tons of wax on that area.
5. NEVER " top-up" the tank when refuelling.

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2007 C 230 Sport.

You might want to try isopropyl alcohol on a clean, 100% cotton terry, followed by some light paint cleaner/polish or swirl remover and see if that helps. I've had some success with this combination on some gas staining. You could also see if bug and tar remover would work.