I paint shiny new GV metal all the time - mostly all exposed to the weather. The manageable pieces and panels I pre-wash with vinegar to etch. Paints sticks fine with no mechanical adhesion prep needed. I cannot vouch for automotive paint however I see no reason to think it would be any different.
Regarding the vinegar....For those who worry about rust we have done plenty with water-based paint as well and have had no rust or adhesion issues. Just make sure the coats are thin.
