Just did a routine OCD inspection of my car and noticed that the clear coat is delaminating (flaking off) on the inside edge of both front fenders. It seems to be pretty uniform, and happening in a pretty much perfectly straight line, maybe 3mm from the edge of the sheet metal. Thank goodness the "lip" is actually about 5-6mm wide, so one cannot see it at all when standing around the car. The damage is only visible from underneath.

Car has been lowered slightly, but I've only used OEM W203 and W209 wheels, so they shouldn't be rubbing. The coloured paint beneath the clear coat seems to be perfectly fine, which is another reason not to suspect rubbing. Not to mention there's no evidence of this at all on the tires.

I'm not even sure how one would even fix this, short of repainting the fenders. Yes the car is over 11 years old, but honestly I expected better than this from MB.

Fenders have not been repainted, ever.

Anyone else come across this? And more importantly, anything I can do to prevent more of the clear coat from delaminating? Thinking of washing those bits, then blobbing some touch-up clear coat onto those areas.

Canadian winters will do that. Salt will eat though any cars' paint. The only real solution is re-painting it. If you remove the fenders and mask it properly, sand the lip (be careful!) and use a good quality clear coat. You could get some decent stuff in a can.
As long as you spray and mask it right, you could fix it. But again, the only real fix is to paint it. Don't pick at it, but I feel like I don't have to say that.

But yeah, I'd say do it yourself. Luckily it's in a spot that's really not visible or noticeable enough that people will know if you messed up your painting. Though it's also fairly easy, just do light coats.

Thanks for the info. Fixed using OEM touch-up paint - comes with both the colour and the clear coat, in 2 little bottles.

To do this repair, I had no choice but to pick away all the loose bits of clear coat with a fingernail. No point painting over delaminated clear coat. Fortunately the remaining (good) clear coat didn't just peel off. It honestly seems like the clear coat inside the wheel wells is too thin compared to the rest of the fender - it's almost as if the paint robot should've done 1 more pass at the factory. Some would say that sanding is more effective, but the clear coat bits simply chipped off, and didn't cause any additional delamination.

After chipping away the damaged clear coat and cleaning the surfaces with rubbing alcohol, I simply coated the affected areas with new clear coat. The trick is to be very systematic and cover everything, and also ensure that the new clear coat joins up nicely with the edges of the existing layer.

Total time was 1-2h labour. Hopefully the new stuff holds. The OEM touch-up paint is of very high quality.

The rest of you may want to check the insides of your fenders as well, to make sure you don't have the same problem.

You did the right thing. You need to seal the delaminated edge of the clearcoat or it will continue.