I think it’s a good idea to paint the balusters in a handrail before assembling all the parts, but the job can be messy, to say the least. Here’s a method I used recently to lessen the tedium.
First I strung a 50-ft. laundry line between a couple of trees. Then I drove 9/16-in. staples into the ends of my balusters. As shown in the drawing, I aimed the staples so that only one leg hit the wood. That gave me a little hook on the end of each baluster to hang on my laundry line
After painting all sides (gripping the staple keeps my fingers off the fresh paint), I hang the balusters up to dry. If I want to get fancy, I hammer a brad into the end of the baluster opposite the staple. Its point allows the baluster to pirouette more easily while being brushed. When the balusters are dry, I yank out the fasteners with pliers and install.
Kathy Damon, Southborough, MA