Zimmerit was a pastelike coating applied to certain German AFVs from December 1943 until October 1944. Its purpose was to create a barrier to prevent magnetically attached antitank mines from adhering to the AFVs. You can read a full account of the history of Zimmerit in the December 2020 issue of MMI.
1 It is best to avoid attaching certain parts of the model before applying the Zimmerit coating. This is particularly important around the hull rear plate; trying to work around the exhaust pipes would be very difficult.