Remove the Base Shoe
If a room has quarter-round molding (aka “base shoe”) at the bottom of baseboards, pry it off and reinstall it later. Here's why: Edge sanding slightly lowers the floor and leaves the baseboard standing on a little plateau. You think you won't notice this, but you will. Edge sanding also scuffs up base shoe, which means touch-up work later. Removing the base shoe sidesteps both problems. Label the base shoe as you remove it to avoid confusion when you reinstall it. Exception: If the base shoe is bonded to the baseboard by decades of paint buildup, leave it in place. If you have newer baseboards and no quarter-round, leave it in place, but expect a lot of touching up.
Pet Stains Are Forever
Water stains usually disappear after a couple of passes of the sander. But stains caused by pet urine often penetrate so deep into the wood that you just can't sand them out. Bleach formulated for wood floors may be worth a try, but the results are often mediocre at best, and at worst, the wood is left pitted and blotched. Often, the only solution is to replace the wood—or finish over the stain and think of it as a permanent memorial to a beloved pet. How do you tell water from pee? Pet stains are darker (deep gray, almost black around the edges) and often look like a map of Indonesia, with big and small islands covering a large area.