Here in sweden it was very comon with the whole underside painted atleast in my hometown in the 70s you had to remove the undercoating to get some finish on the paint. You had to have the same finish on the underside as on the topside of the car othewise you was lame
Gonna do much the same on my '37 g***er coupe, all old time trends on it My aim is to keep EVERYTHING on it pre-'70.
My 62 Chevrolet SS back in the day (1966). Painted the complete underside with white gas station island paint. My Dad who was a man who kept his car very nice told me "everybody thinks I'm nuts the way I keep my car. Son you put me to shame",as I crawled out from under my SS with brush in hand!