It would be best if the factory spent extra time and money to make sure everything was painted/coated to begin with – but in art of pinching pennies, it is a wonder that most items even got what they did get (paint or coating wise).
I have seen new leaf springs, differentials, control arms and spindles come from part suppliers without any paint. I have seen the same with a dusting of paint (if you want to call it that). I have also seen them with a nice layer of paint.
Things like leaf springs and differential housings, rust, although not good, is not going to be a big concern. Control arms would be more susceptible to rust – but not to same effect as frame rails are.
The body (frame rails, floor pan, trunk pan, quarter panels, etc.) use thin metal and rust (cancer) will affect those items to point of disabling car long before rust will effect leaf springs or spindles.
Now with that said, rusty control arms might be the next thing to cause a problem.
I’ve heard a rumor, that on a calm and quiet fall night, you can actually hear your neighbors Chevy rusting.
BudW