Nice skin
Just as a note of nomenclature, though, Tarnmatte was an
anti-radar coating and Alberich was the anechoic stuff. Tarnmatte was only really used on snorkel heads, as they were the biggest radar reflector when at periscope depth. It also was made of rubber and rust, so I imagine it had a reddish appearance.
ref:
http://www.uboataces.com/sonar-coating.shtml
Alberich, on the other hand, was black rubber that was tiled all over the boat and probably looked a bit like this:
To do a weathered alberich skin, I would do some peeled off tiles and some rust in the gaps, much like a lot of the Soviet SSNs of the 80's.
Anyhow, nice job and keep up the good work!