This homeowner had wanted white spindles on the railing since the home was built years ago. The builder didn’t do it.
It is easiest to paint the spindles before they are installed but it is not impossible to do after the fact. Make sure that you take time to tape carefully. It can be tedious but it will make a difference in how the project turns out. Also keep in mind that it won’t be perfect. It is difficult to get every little nook and cranny. Next sand the varnish coating off as much as possible – the small areas are the hardest. Then vacuum and use a tack cloth to removed the dust. Now get out the primer. Depending on the stain color, you may want to do two coats of primer. Next, paint the spindles. I use Sherwin-Williams ProClassic. It comes in latex and oil base. The latex drys as hard as an oil base but is easier to clean up the tools. I recommend two coats of paint. Lastly, remove the tape carefully and completely. I use a wallpaper knife to help me get those tiny pieces off.