I too have done many many years of underside detail work and research and came up to an almost perfect conslusion. All cars built in the San Jose and were not ordered with the underside sprayed with undercoating (unfourtunatly it was an option back in the days) were painted red oxide, which is the orangish red your refering to. However car built in Metchuen and Dearborn, were painted in a low gloss black. I have seen some late 68 Metchuen cars painted in a GI joe green, that can be found on the new year 69's. But contrary to what most may think, these cars, when not optioned to be sprayed with undercoating, had red oxide sprayed, as well as low gloss black. For a "true" concours restoration, the underside must be painted in the correct factory color, (for example) car from san jose comes with red oxide, opted with no undercoating, should be painted red oxide from the front frame rails (starting in the middle of the torque boxes) and painted all the way to the rear gas tank, laving the rear frame rails low gloss black. But remember though, the undersides were put together before the cars were painted, so yes if you want a perfect underside resto, you will need to have the cars original exterior color, oversprayed underneath the car. Personally i dont have this done to my car mainly because i think it looks like the car was never taped off underneath, and a lot of people dont even know thats how they came from the factory, so i left it out, although my dads is done correct, only him and i know that it is
But like ive posted in the past, different strokes for different folks, if you like red oxide and your cars from dearborn, go for it, you actually dont loose any points if a judge sees it because all colors are accepted in the a MCA rules.
What factory underbody paint have you seen?
