im pretty sure the rear quarters are steel. Because like many W215 both my cl500 and cl600 have rust bubble spots appearing at the top of the quarter panels where the roof meets the quarter panel . If it was magnesium or aluminum these rust spots would not appear .

i think the roof is steel too . not sure though.

I do know the trunk lid and front fenders are plastic .

in 2006 user 03cl55amg posted a comment here
https://mbworld.org/forums/cl-class-...body-made.html

supposedly
a dealer brochure listing these metals :
data from a dealer brochure for my 2003 CL55 AMG and here is what it says:
Front Hood: Aluminum Sheet Metal
Front Fenders: Polymide blend of materials-unreinforced thermoplastic to reduce damage from minor impacts
Trunk Lid: Two fiberglass reinforced duroplastic shells bonded together
Gas Tank Lid: Same material as front fenders, thermoplastic
Rear Fenders: Aluminum
Doors: Inner shell magnesium, outer shell is aluminum sheet metal
Roof: Steel

take a magnet to all the panels and see if it sticks anywhere

galvanic(?) corrosion can still happen to aluminum. I work with aluminum gutters all day, you see this most apparent on aluminum gutters where someone uses steel screws instead of aluminum rivets.

In the case of non-ferrous metals(?), it’s not called rust, it’s just called corrosion. You’ll notice if you pick away at the bubbling paint, the corrosion is a chalky white type of corrosion. Not like steel rust

however, we can easily test this as you say, with a magnet. I’ll report my findings

I do believe the roof of the cl500 to be magnesium, you’ll see the same chalky white corrosion instead of brown rust. A quick google search says magnesium is the most lightweight structural metals. HOWEVER there are parts on the cl55 that are steel instead of aluminum. The rear subframe immediately comes to mind. I can’t imagine why the cl55, a more expensive car, uses cheaper metals?

Perhaps they anticipated some adventurous 800+ horsepower CL amgs to be developed lol

one thing that really puzzles me, is the rubber gasket between the roof and rear fender. If they were both steel as you suspected, this rubber gasket is unnneeded, because the metals would not need to be separated ...

mercedes knew the roof and rear fenders are dissimilar metals , hence why they used this rubber gasket here and no where else between body panels of the car

in short, mercedes knew this issue would arise

im just puzzled why is it happening still with that rubber gasket seperating the dissimilar metals? I’d understand if I lived in an area where they salt roads in the winter. But my CL is a Florida car with no rust otherwise

edit- Wow , per the into in your comment, I have realized the exact issue here

the second biggest corrosion complaint on the W215 behind the rear fender, was the bottom of the doors

guess what is also magnesium and aluminum like the rear pillar/rear fender combo? It’s the doors , aluminum shell with magnesium structure

so obviously something Is wrong with mercedes magnesium or aluminum supplier in the 2000’s.

Lets keep this thread updated with any experiences with repairs

PS sending your shipment sometime this week!

this is a cl55, no? I’ve read the rear subframe(3) is aluminum on cl500.

otherwise, looks like the roof(7) is mostly aluminum with some STEEL bracing/support. I can’t imagine the aluminum roof(14) and aluminum fender(4) are causing issues

the difference between steel/aluminum from cl500/cl55 makes me wonder how much less steel is used elsewhere on the cl500

its funny how (7) is both steel and aluminum, I’m sure they have some sort of buffer/barrier to prevent corrosion but it makes you wonder if the tiny bit of rust we see on the rear fenders-
is actually indicative of some really bad corrosion issues under the roof cover (7) where the steel is...

either way, that diagram is a huge help slonar