Refinishing stairs is a project that can take days, and you can't use the stairs while it's in progress, so it's good to take advantage of any shortcuts you can find. Professionals often have to refinish stairs as part of a larger floor refinishing job, and some of the tools they use can make the job easier.
You also can save time by finishing with oil, which is much easier than polyurethane to touch up.
Strip the Finish, Don't Sand It Off
Even though stairs treads form a flat surface like a floor, it's a mistake to try to sand them with floor sanding equipment, or even a belt sander. It's too easy to lose control of these machines and end up with damaged wood.
A faster and easier alternative is to strip the stairs with an infrared paint stripper. This tool generates enough infrared heat to loosen the bond of the finish to the wood, but not enough to damage the wood itself.
It makes the finish loose enough to scrape off without generating noxious and dangerous fumes.
Scrape With a Pull Scraper
When they need to remove finish from corners or under cabinets, floor refinishers often scrape it off with a pull scraper, and one of these tools is ideal for stripping stair treads and risers. You need one with a blade that is 3 to 4 inches wide, and you should keep a belt sander with a 120-grit sanding belt handy so you can keep the blade sharp by grinding it on the sandpaper.
When its blade is sharp, the scraper easily pulls off finish, even without the stripper. It also gets into corners and other tight places.
Always scrape with the grain of the wood -- never across it.
Wash With a Solvent, Then Sand
After you've finished stripping finish off any wood, there inevitably is some residue left behind to gum up the sandpaper. You can get rid of that residue by washing the stairs down with a strong solvent, such as lacquer thinner.
It cleans the wood and pulls the old stain out of the grain without raising the grain. After this treatment, you should only have to make two passes with a palm sander, using 100- and 120-grit sandpaper, to get the stairs ready for finishing.
Finish With Oil
Finish wears off stair treads even faster than it wears off floors, and after a few years, there's usually a significant difference between the middles of the treads and the edges. If you finish the treads with polyurethane, you have to strip the treads again if you want to renew them.
An alternative that's easier, both at the time of refinishing and several years down the road, is to apply several coats of wiping oil. It penetrates and protects the wood from inside, and when it does wear, you simply freshen it up by wiping on another coat.