If you dont know what type of wood and finish is on then first find out
, going in blind you are very likely to just make a mess of everything.
Light wood ? Pine and Birch need a prestain treatment, oak doesn`t.
Without prestain on pine or birch it will look like crap. A colored poly
needs a pro to do it even. Get a few bids from wood refinishers and
learn before you start. Stair railings are detailed intricate work,
cross grain sanding or a poor staining can ruin a job making clear or
paint necessary. You realy have to learn about wood refinishing before
taking on such an important piece. Many times ive seen homeowners so
proud of their wood job, only to inside laugh at it, knowing it needs to
be redone, or is ruined needing paint. Only a pro seeing it can help.