Factory didn't spend any time or much care, and painted those installed.
Me, I like to paint the backsides & hidden areas, assemble, then paint the outside complete in the OEM scheme.
I also prime everything after cleaning it to death. Factory didn't do that, though. Nor were they real shiny, more like a semi-gloss.
Every one of them I've done over the last 40 years still looks great, though I do keep them clean & leak-free.
Dead-nuts correct is to paint it like the factory did, a quick hosing with paint with no regard to longevity.