zimmerit

Zimmerit was a paste-like coating used on mid- and late-war German armored fighting vehicles during World War II. It was used to produce a hard layer covering the metal armor of the vehicle, providing enough separation that magnetically attached anti-tank mines would fail to stick to the vehicle, despite Germany being the only country to use magnetic anti-tank mines in numbers. Zimmerit was often left off late-war vehicles due to the unfounded concern that it could catch fire when hit. It was developed by the German company Chemische Werke Zimmer & Co (Berlin).

For easy identification: if the tank has weird small grooves along it's skin it's probably zimmerit.