This is how I undercoated my Ork Wartrakk.
Some people use black undercoats when painting vehicles and some use white. For some of my Ork vehicles I use both! A white undercoat for the bodywork, to enable me to use lighter colours or reds and then black for the mechanisms and other metallic and rubbery parts.
The model was built with the crew left off for painting separately. The Big Shoota stayed with the gunner. I used some of the Ork glyphs from Forge World, I would have preferred if the glyphs were cast in a similart vein to the Inquisitor purity seals, but they’re not, they have quite a thick backing.