Märklin metal bodied coaches

[Swissrail]

I have just bought the Märklin 4228 Rheingold Coaches set. Although it is beautiful, it needs weathering to really bring it to life. I have been told that these coaches have what was described as "tinplate" bodies although I cannot imagine such an antiquated technology being used to make models these days. As such, I am told, the livery was printed using a lithographic process which is "very easy to destroy". Is all this correct and if it is, how do I weather these coaches without ruining them?