Dart underbody paint

The body was dipped in a vat of rustproofer/primer to about 6 inches from the bottom of the windows. Look at an old Mopar that the trim panels are missing and you will probably see a line like a highwater mark from Katrina.
During painting, any paint that was between the rocker panels was just overspray.
They shot undercoating in the wheelwells. If the car was coded for undercoating, they shot the bottom of the car (or most of it).
After a couple of years the undercoating dried out (like an old asphalt road in the desert) and left all sorts of nooks and crannies to retain moisture. It was more of a sound deadener that anything else.

As they say, drive your brand new Chrysler home, park it in the driveway and watch it rust!
A-Body owners will attest to that---tops of front fenders!