rust or faded factory rust proof?
 

Do you think the brown area in this picture is rust? Or faded factory rust proof?


P.S.:
Anyway, when you degrease the engine room for engine detailing, is the MB factory rust proof removed too?
If you have any information about compatibility of aftermarket rust proof with MB factory rust proof, let us know.

Rust being iron oxide . . . get a magnet and see what happens. It does look like rust.

As far as Mercedes "rustproofing" I'm aware that during the manufacturing process there was a liberal use of zinc primers and in areas where the primer was going to be ineffectively applied, they injected a waxy substance as a moisture barrier.

The vast majority of the yellow is cosmoline but it definitely looks like there is a small section of rust. Is the area coated or does it flake off?

It more looks like coating.