I have been under a lot of new cars and I have seen them made all three ways mentioned (above): light paint (which is inconsistent), bare and bare but coated with cosmoline – with no rhyme or reason.
I have even seen a new car with one arm painted and other side not.
I “think” cars used for dealership showrooms or show cars, got an all-out paint treatment. Again, this is “think” and no facts to back anything up.
I also “think” cars going to some regions got nothing (like to Arizona, where rust is a non-issue). On parts coated with cosmoline, some would be barely touched, and others coated ¼” thick of the stuff.
I don’t have any answers as to those trying to make them look like new.
All I know is: a cleaned and prepped part, which is painted well, will look nice for a long time. Also painted parts are much easier to keep clean.
I prefer to blast and paint my suspension/steering parts black (a few thin coats) when rebuilding them and have had a lot of luck that way (even for “beater” cars).
For my current F & M cars, I hadn’t rebuilt the suspensions, yet – but on those, I will be installing these reinforcement plates to them:
These are made by AR Engineering (part # AR053), but appear to only be available from Firm Feel (part # LCAP4).
This is a picture of Firm Feels lower arms after reinforcement plate is welded on (worms-eye view):
BudW