Hey, we know they were made of pressed steel, with a lining that allowed transport of water; and Mr Googles says it was galvanized steel, however, I was not able to corroborate that.
If indeed it were galvanized, no need for primer.
If you look at photos of wartime cans that haven't been refinished, there us much more corrosion than there would have been back then.
When I paint mine,
I like to vary the colour of the cans a bit, as they may have had more or less exposure to the sun, and may not have all been painted the same depending on date of manufacture, the most obvious being some still in Dunkelgrau.
I add a bit of chipping in a dark colour, sometimes fuel spillage,
with random wear on the edges that would have been exposed to damage.
A bit of Tamiya 'titanium' weathering rubbed on some edges to simulate 'worn to metal surfaces'.
If you add a dusting on top of all that, no one is going to notice primer or not
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