Zimmerit was a cement-like paste applied to German armor in WW II for protection against magnetic mines. Any reference books on German Panzers will show it on many tanks, mostly Pz IV, Panther and Tiger family. There are a couple sites in Germany and England that sell the zimmert kits that you can glue on to the tank. Otherwise you can use modelling paste and smear it an yourself, then use a screwdriver blade and make the marks. I just got one of the stick-on kits in the mail yesterday and it looks nice, but haven't had time to put it on yet. Here's a couple photos of a 1;35 Tiger 1 model with paste and screwdriver work.