Your GB 176 in the picture looks the way it was when you found it. My money says it does not look the way it did in 1939 the last day it was in operation. Back then, like today, good business is about appearances and I can't imagine even an out of the way country station getting much business with equipment looking like that.
For your purpose, wire brush all of the flaking crud and brush on a coat of rust stop primer. Then paint it with your choice of colors. You can thin the paint with gasoline to flatten it up. There is a lot of tricks to "weather" the look. (just before the paint is totally dry, stick your hand in the dirt and then rub it over some spots ) Check with some of the guys that have done rust orations.
