I sanded back part of the stairs and put in a new bannister. I was going to varnish it, but decided in the end to treat it with Dutch oil. It gives a lovely warm finish to it, and it's a doddle to apply. You do have to top it up every year or so, but that burnishes it and makes the colour richer. I also have all oak doors and must say it matches in very well with them. As for sanding, I used a sander 1st and finished by hand. If you get it smooth and then apply the paint I think you'll be alright.