used to work at Land Rover axles Perry Barr at the time of the launch of the lightweight and Metro the Michael Edwards era when crown wheel and pinion were lapped in a quality sets.
They used to use a black paint I recall called sunkarite (don't know if the spelling correct) but you could bake it and smack the hell out of it would not chip .
Cellulose based I believe, the axle cases,swivel housing etc were dipped in it. probably got too expensive or they reverted to water based paint when they moved the lot to solihull