A couple of days ago, right after the big snowstorm, I noticed some snow and a little dust on a quilt on a '50s walnut sideboard out in the barn. After staring stupidly at the roof for a while trying to figure out where the hole was it dawned on me that a critter had slept there. And that it was wet. Pulled the quilt off and sure enough there on the top was the mother of all white spots. After cussing a bit I went on about doing other stuff and later noticed cat prints in the snow leading into and out of the barn, and no other critter prints, so obviously a cat got in and found that to be a nice place to sleep, and it being snowing and all he was covered with snow that he brought in. So it was a cat-sized white spot.
Well, we had some Restor-A-Finish on the shelf and it says that it will if used with steel wool remove white spots. So I tried it. At first nothing much happened, then all of a sudden a patch turned walnut color. I kept at it and after 45 minutes or so of work I had the whole patch back to normal color. So I wiped it down expecting the shellac to be gone, but it wasn't--only difference between that spot and the rest of the top was that the place I'd been working on was a good deal shinier.
So the stuff (Howard Restor-A-Finish--stocked at the local hardware store) actually works as advertised.
Now to figure out how to encourage the cat to sleep somewhere other than on the good furniture.