How to paint balusters or a balustrade.
What is the best method - brush, roller, sprayer - for painting balusters?
They are not yet installed.
To start, it would depend on where your balusters are going to be situated. If they are intended for an interior project then I would most likely spray them.
Exterior balusters on the other-hand (in my opinion anyways) should be brushed or rolled. When paint is used, I myself prefer rolling them.
Before you do coat them, be sure to use a product that's specifically designed for your particular environment or climate (rain in your case).
The reason that I recommend (brushing or) rolling an exterior balustrade is that it tends to make the product stick better (pushing the coating into those microscopic nooks and crannies). Sticking is a good thing especially when dealing with extreme weather conditions.
Even so, spraying would be much faster though and could still end up lasting a number of years. It all depends on environment, proper surface preparation, how well your coating product adheres and the type of wear and tear the balusters have to go through.
Last but not least is to also keep in mind the kind of surface being coated for both interior and exterior balusters.
You will have to decide on the process etc. that works best for your particular situation.
Your local paint retailer will be able to help you decide on the best, process, tools, products and materials to use for the project at hand.