There are a lot of different valid techniques to replicate the pattern of the Zimmerit coating on a scale model. In this brief tutorial I'll describe the one I used on the model I'm currently building.
The Zimmerit was a coating used to cover some parts of the hulls of German armored vehicles during the Second World War. Its purpose was to avoid that "sticky" and magnetic anti-tank charges could stick to the armor.
A bit of history
The use of the Zimmerit (from the name of the German company Chemische Werke Zimmer & Co that produced it) began in summer 1943 and continued until September 1944. It was applied on all tanks and closed-cockpit armored vehicles, seldom on light vehicles.
It was a paste made of vinyl glue, pigments and sawdust, laid out onto the vertical and sloped parts of the armour. Before drying, a rough pattern was applied on it, in order to diminish the surface on which the "sticky" bombs could adhere. Due to the thicker layer of non-magnetic material, moreover, the magnetic charges were put too far from the metallic hull to remain attached.
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I started sanding the desired part with a coarse-grained sandpaper, thus creating a rough surface for a better adherence.
I used fine Milliput bi-component putty (the white one): I laid it out on a smooth surface, using talcum powder to avoid it to become sticky, and flattening it with a plastic tube until it became a thin sheet.
Then I laid out the sheet on the model surface, pushing it with a wet putty knife and cutting out the excess. I helped myself with a masking tape to delimit the areas on which apply the putty.
Once the putty adhered, I rubbed on it some talcum powder to avoid it to become sticky, then I made the pattern visible in the photo below with a flat-head screwdriver held oblique. This technique isn't much different from the actual one! Anyway, there are several variants of it, depending on the vehicle.
This is the final effect of the Zimmerit coating on the scale model I'm currently building.