Bonnets are obviously done separate as there is paint beneath where the hinges sit.
Most cars (Holdens) I've pulled apart seem to be painted in pieces as the paint is on both surfaces that overlap, but I've got a lot of old (HQ vintage) brochures and books that show cars being painted in one piece on the production line. These may be staged pictures, but I doubt it. I'm pretty sure that they are painted with doors in place, but front guards and nosecone don't seem to be.

Anyone......?