This was discussed before, but then the video got deleted. They added it again.
This film (it says 1956, but the final footage is of a 59, so it's probably closer to 59) has shots of the engine being painted.
https://www.youtube.com/watch?v=dmwdFTyMmiQ&t=684s
The automotive part starts about 16:25. Lots of interesting shots including blocks, cams, frame rails, etc.
17:50 - inspector stamp on the block
19:40 - block painting, including painting three different color blocks.
23:30 - How about that frame? The complete exhaust system is painted!