I don't think it's too practical to paint them in place. A lot of railings are one catwalk width away from the deck structure behind them, and while you can see the brass on the inside, you can't get at it with a brush. And, usually the deck is a different color than the railing, so airbrush is out without some complicated masking.
Like most things, it's a combo. I airbrush mine before assembly, install them, and then come back with railing color paint and a fine tipped brush to clean up and misses, bends etc.