The rails are the most visible parts of a stairway, so it pays to paint them with an eye to detail. That can be challenging when it comes to the outsides of the balusters, however, because they are often so close together that you can't get your head through to see what you're doing. It's not a good idea to paint them blind, because any voids or blotches you leave will be immediately visible from the other side of the railing. The solution is twofold: Lengthen your paintbrush and extend your vision with a mirror.
Spread a plastic drop cloth under the bottom rail, if there is one. If the balusters connect to the deck, stairs or outside of the staircase or balcony, tape the horizontal area around each one with painter's tape.
Paint the outsides of the balusters before you paint any other part of the railing, because you need to lean over the railing and reach through the balusters to do this part of the job. It will go more smoothly if you have two high-quality 3-inch paintbrushes with angled bristles at your disposal. Tape one of the paintbrushes to a 2-foot roller extension pole, using duct tape.
Lean over the railing and paint the bottoms of the balusters with the brush that you taped to the extension pole. Paint the outsides of the balusters up to a height that you can reach by hand with the other paintbrush. If you need more height to see over the railing, stand on an empty 5-gallon bucket. Don't give yourself too much extra height, or you may lean too far and lose your balance.
Finish painting the outside of the baluster, up to the underside of the top rail, with the other paintbrush. To get the part right under the railing, hold a hand mirror between a pair of balusters to help you see while you extend the brush through the balusters with your other hand.
Paint the other three sides of the balusters, the top and inside of the bottom rail and the underside of the top rail from inside the baluster. Use the mirror to help paint any parts you can't see, such as the outsides of the rails.