Thanks for your suggestions on chemicals and warnings against drum sanding. I went shopping Monday morning for chemicals, and in the meantime Mr. Weedy made faster passes on the floor with the sander to avoid heating up and globbing the shellac or whatever was on the floor. Results: Denatured alcohol did nothing. I tried wiping with a rag, scrubbing with steel wool, flooding a small area and letting it sit, all with no results. Guess it's not shellac. I then tried lacquer thinner. Scrubbing with a rag seemed to have an effect, so I did a fairly large area. Once it dried, however, there was minimal results, and less so on the unsanded wood than sanded. I tried it in one of the bedrooms that had a single sanding pass, and quit once I started feeling a little stomach cramping. :-) Yes, I had the windows open and a fan going. Sanding that didn't seem any different from the area I didn't apply the lacquer thinner. Didn't try the mineral spirits yet. In the meantime Mr. Weedy had sanded one of the bedrooms about 18 times with 60 grit (seemed to create more dust than 20 grit), and made headway. I took an orbital sander with 40 grit to the spots left behind (slightly deeper grooves/dents) and here's where we're at: Bottom left corner is where I hand sanded. Lowe's had no oxalic acid, but we've got some in our garage 100 miles away, so we'll work on the kitchen again later. Also, in the meantime, we used a belt sander on the mosaic inlay portions to get off the lippage, machine sanded over it to even out the belt sander marks, and then chipped away with a hand sander to get the residual off. This shows what our eventual goal is, but it'll be dozens of hours if we have to hand sand off this finish. I've done some more googling and may try citrus strip or Formby's furniture restorer to remove the finish. We've also got this area that had tile over it. The thinset seems to have bleached out the wood where it touched, but even aggressive hand sanding is having a tough time evening out all the discoloration. I'm still very open to additional suggestions to attack this. And I must say that I now know the answer to the question, "why would anyone cover up hardwood floors?" Anyone who asks has never personally refinished them. :-)

Thanks for the feedback. There is a large enough closet that maybe a board could be pulled from, if that's needed to match the old wood. I hope the guy who refinished the floors could do a good job with this. He did a nice job with the refinishing, and has a good reputation locally. I think that's who the contractor will want to use, and he is covering the fix so I don't feel like I should be too bossy about who does it. Feeling nervous about it all.

This looks like top nailed thin slat flooring. It is most likely square edge with no tongue and groove, and was 3/8" thick originally. That is my best guess. Whether or not that is true, my advice is to inspect every square inch for any nail heads that are close to the surface of the wood and to sink those below the surface before you sand. You will then need to sink any nailheads that become exposed during the sanding. These I call 'shiners' for obvious reasons. Be careful in what 'putty' you choose to fill any holes. Some wood fillers will seal the wood surface and prevent any stain from penetrating the wood; you could end up with an unsightly end result. You need to use a wood filler that you can then sand flush with the wood surface during your final prep and be confident that you have not sealed the wood, nor left any stray. Not all wood fillers take stain properly, no matter what the manufacturer claims. Test everything for compatibility before you commit to your product selections.

You can purchase on Amazon, search for "Bostik adhesive remover wipes" The other alternative that we use on site would be mineral spirits. You must follow up with a damp cloth after using mineral spirits. If you do not clean up with a damp cloth, which will leave the mineral spirits on the surface of your floor, it can tarnish your finish/create a bad haze that can not be removed.