After spending too much time on the first step, trying to sand away many layers of 30 year old chocolate brown stairs, along with the knicks and dents in the wood, we got smart and used a chemical paint stripper. Many of the "safer" ones say they work in as little as four hours, but I chose Back to Nature brand Hi-Speed Ready-Strip
which says it works in about a half an hour. The half gallon container was more than enough.
1. Open your windows and turn on a fan. It might be "low-odor and environmentally safe", but it stinks.
2. Shake the bottle really well - it's very separated.
3. Pour some liquid onto the top step. Using a cheap paintbrush, spread over the step in a thick layer. Make sure to get the corners and edges well.
4. Move down the steps to the bottom, and throw away your paintbrush, then find something else to do for about 2 hours. I started the bottom step after a half an hour, and worked my way back up. But during the rest of the paint removal process, the upper stairs that had had the stripper on them for about three hours took a third of the time it took to remove the paint from the lower steps.
5. Using a paint scraper, scrape across the step with the grain of the wood. The old polyurethane comes up with the stripper goo. Wipe the goo off of the scraper, and continue (don't bother trying to rinse it off). I scraped it into an old shoe box, but where doesn't really matter. Scraping takes off most of the stripping agent, I didn't bother to wipe off the remaining.
These steps are old and dinged up, and the dark stain penetrated into the wood, and it's imperfections. I was removing the many layers of old poly with the chemicals in order to get closer to the wood and to get started. The picture here shows the stairs AFTER the chemical stripper was scraped off. It was only the first step of removing the old color, but it made the rest of the project move much more quickly. The time (a day) and cost of the stripper was less than the time and cost of using sanding pads to get through that old poly. Not to mention - much easier on my back. I wore work gloves for all of this step, I really didn't get any on myself.