Chrysler's dip process started with the 1960 models and the beginning of unibody construction. Each body was dipped six times in rust resistant chemicals and rinses. There was an additional seven spray processes which included two coats of primer and it was all topped by two coats of enamel. Chrysler's dip process reached the belt line. Only Rambler went to the roof. Dipping the car's paint colour would have been a very costly process. They would need a bath for each colour. In the case of a 1962 Plymouth, that would be 13 baths of paint. And if the colour in a bath had to be changed, the bath would have to be drained, cleaned and dried before the new colour could be poured into the bath. Otherwise the old colour could mix with the new. Imagine remnants of purple in a bath of white.
Chrysler stopped painting the underside of bodies with the 1952 models. Chrysler was on a cost-cutting spree and painting the underside of the bodies was one of costs eliminated. "Science and Mechanics" magazine in their test of the 1952 Plymouth noted the lack of paint. They had tested a 1951 Plymouth the year before.