Old painter shared one of his tricks with me.  Put a tight fitting plastic glove on, then a brown jersey glove over that.  Pour paint in a 'paint tray', then dip the palm and fingers of your hand in the paint and ... paint.  When finished, throw the gloves away. 

Thats how he painted all smooth rails, or stair ballasters.  Takes some trial and error practice to get good coverage, but worth the effort.  Not many drips if done right and fairly quick,  efficient you might say.