I don't know if modern techniques apply to this, but I work at a job shop that builds a bunch of different things. When we paint, let's say a trailer, we first slag blast the thing, then prime, either red or grey oxide. Then the top coat is applied to the visable parts with a light dusting to the "undersides". Typically overspray. Prime definately shows through. It would seem that in a time of war when your stuff was getting blown up as fast as you could make it, paint would be the last thing on your mind. Then the field crews would paint to suit there needs. I wouldn't be surprised to see : light paint, runs, sags, orange peel, fish eyes, etc, etc. I never thought about it until now, but the underside of the hull (Very hard to rotate after assembly) would possibly be no more than primed. This was heavy plate and surface scale would not be an issue on a vehicle that had such a short life expectancy. The fenders and such would probably never see a top coat. IT's too bad they didn't have Core 10 steel. That's the stuff they use to build art and fancy foot bridges. It oxidizes to a certain point and then the corrosion becomes it's protection. That would be a cool modeling project.
Just my $.02.