In the latter half of the Second World War, magnetic mines were common infantry anti-tank weapons. These mines were thrown or placed on tanks and relied on strong magnets to attach, then blasting through the armor with a shaped charge. They were particularly effective against German heavy armor, which were often relatively helpless against infantry assault - although the magnetic mine was a close-range weapon and, as such, very dangerous for the infantryman to use, the Soviets had enough manpower not to worry too much about losses, if they could be exchanged for some German heavy tanks.
In 1943, the chemical company Zimmer developed a paste containing the following:
When applied on armor in sufficient quantities, it prevented magnetic mines from attaching.
On December 29th, 1943, OKH (the army high command) ordered the following vehicles to receive Zimmerit coating:
However, vehicles other than Panzers rarely received Zimmerit coating. It was also applied to most late war heavy armor, including
Zimmerit was applied on all surfaces of the hull and superstructure. The turrets and sideskirts were not to be coated, but were often anyway. Zimmerit was applied in two layers. The first layer, 5 mm thick, was allowed to dry for 24 hours. The second layer was 3 mm thick and its surface was formed into an uneven pattern with a metal comb. After drying for four hours, it was hardened with a blowtorch. (Without a blowtorch, it took eight days to harden.)
Zimmerit was very useful in protecting against magnetic mines. It also helped conceal vehicles by making their surfaces less reflective. Nevertheless, on September 9th, 1944, OKH ordered that the use of Zimmerit was to cease. This was apparently due to untrue rumors saying Zimmerit could be ignited by non-penetrating hits, causing the vehicle to be lost.
Zimmerit was used for some time after this, however. By this time it was no longer as useful as it had been, as the Allies widely employed anti-tank rocket launchers and other less suicidal infantry anti-tank weapons.
Sources:
http://www.activevr.com/afv/zimmerit.html
http://www.panzerworld.net/Zimmerit.htm
http://kotisivu.mtv3.fi/pkymasehist/onteloammus.html (Finnish)