At the end of the summer I went on vacation and left my car parked, outdoors, at the airport. I was away for about 18 days and when I got back the discs were covered in rust. Normally after a few miles of driving/braking the rust gets removed by the brake pads and everything returns to normal.

Not this time. After 3 weeks and 2,000 kilometers the brakes were shuddering and the steering wheel was moving left to right under braking.

The dealer replaced all 4 discs under warranty. While that is a good thing, I'm guessing the next time this happens I won't be so lucky to get it covered. I have noticed that the discs tend to corrode much more than with any other vehicle I've owned. Anyone have tips/suggestions on how to avoid corrosion (yes, parking indoors or under cover would help) and/or how to safely remove the corrosion when it's bad?

We have a machine to scrape of rust from discs and to straighten them.
Maybe there is a shop in your neighbourhood that can do it?

That's an interesting one, I've never heard of discs corroding that badly after leaving the vehicle outdoors for only 18 days

I once had an old Holden parked outside in my parents front yard for over 3 years and the discs had no probs using it again after that time, were you able to get any info off MB as to why it's happened ???

Just thinking about it was the vehicle left in a highly corrosive enviroment, eg. near the beach or similar as salt in the air from the ocean could cause that kind of damage.

I don't park outdors for more than 5-6 hours a day, but good that they took care of this for free. Acid rain effect? I never heard about such problems, most people are parking outside.

This happened to me on my E550C. It has also happened to a ton of other E550 drivers, I believe one of the members here has started a class action law suit. See if you can find the thread in the W212 forum.