Start by completely removing the old finish. Old paint can be removed with a wood stripper and cleaned with minerals spirits. You need to apply enough stripper to keep the surface wet. Otherwise, it will evaporate and dry out the wood. Make sure you give the wood stripper enough time to do the work of loosening the paint before you scrape it off. Try and do all the stripping in one session. Otherwise, the paint will re-harden and the process will need to be redone.
The wood should be sanded to remove any blemishes or rough spots. You’ll save yourself a lot of work by using a power sander (Image 1). Use 100-grit sandpaper to start. Exerting pressure slows down the sander and makes it less efficient.
A profile sander can do the job in hard to reach areas (Image 2). A couple of basic rules for sanding: Work with the grain, not against it. Sand the banister at least three times, switching to a finer grit with each pass.
If the banister is made out of soft wood, apply wood conditioner to help it absorb the stain evenly. Gel stains will drip less than liquids. The longer you leave them on, the deeper the color will be.
When applying polyurethane, a hair dryer on the cool setting will help break up any air bubbles. Allow the stain to penetrate for five to 15 minutes before wiping off the excess. Apply additional coats if you want a deeper color.