At one time the practice of thoroughly cleaning castings and machined steel parts, washing them with kerosene, wiping them dry and then whitewashing the parts of interest was commonly done in shops.
This allowed the light kerosene oil that had seeped into any cracks which may have not been visible before, to wick out into the whitewash and thus clearly display their location.
Probly don't have anything to do with the white driver tires in the erection photos of locomotive works products, but an interesting and
common technique for finding cracks nonetheless.