Q: Before applying finish to my fir flooring, I want to know whether my fir floors have been properly sanded. There are numerous areas of darker color, for example, a 3-inch band of darker wood parallel to the wall and offset by about 6 inches. It seems like this strip simply was not sanded as much as it should have been. Other dark areas with pretty well defined edges are in the middle of the floor.
How can I be sure these are sanding issues and not due to some other cause?
A: Try a hand scraper on a couple of them and see if you can scrape it lighter. If not it is just resins or some extractive in the wood itself and not old finish.