1970 Duster factory chassis paint details.

try and think "assembly line"
it would be very difficult for the line workers to assemble anything with painted threads or adjust and fit anything slopped with paint...those parts would be left uncoated.
all wheel wells were undercoated except some California cars which were blacked out just with paint
floors and frame sections were primer with some body color over spray
lower control arms were cosmoline (orange waxy oil coating)
there are guys on this site that know much more than me.