One thing you need to remember, if you paint them off the car, let the paint gun "see" them as you will see them on the car!
Don't make the mistake of hanging them and painting them so when you put them on the car there are funky spots where you see them, and the back side looks perfect.
And you are rolling your eyes because of the obvious, think again. I have seen guys do this over and over. I have painters I work with who will do this over and over and over. They will paint a hood hinge and then after it's on the car have to spot paint some bare spot on it.
It is simple, hang it so it is OPEN as you would see it on the car (you don't see your hinges when they are closed right?) AND hang it so you are spraying it as you see it on the car. Sure, get the back side all nice and all too. But if you let the paint gun see it as you will on the car you KNOW it will look good.
Brian