Hi all,

I'm refinishing the flooring throughout an older house (1930s). My goal is to sand flat and clean, then seal and finish (no stain).

The flooring is 3/4" t&g, and appears to be white oak. It's probably as old as the house, and had been covered with wall-to-wall carpets or linoleum for the last 30+ years.

After removing the carpets and linoleum, I found that several rooms had the "postage stamp" look -- like they once had square rugs affixed to the floor, and only the area around the square rugs had been fully stained and finished. One of these square rug areas may have had a top coat, but if so it was very light:

https://i.imgur.com/uZEqcxa.jpg

I'd never seen this before, but at least there's less nasty finish to gum up the sandpaper, I thought!

After two passes with 36 grit and one pass with 60 grit (drum sander), all of the finish and stain had come up, but the rug squares remain dark and blotchy compared to the rest:

https://i.imgur.com/TaKAaqs.jpg

This is especially apparent at the "edges":

https://i.imgur.com/3x1WTBM.jpg

It's strange, because some places the blotches seem to carry from one plank to another, but in other places an entire plank is relatively clean.

So is this simply natural discoloration or aging of the unprotected wood? Or is this from liquid staining?

More importantly, what, if anything, can I do?

I'll note that these blotches did lighten after the 60 grit, but only a little bit -- so little that I am not confident that another pass would make a lot of difference (but am ready to try).

I also tried diluted oxalic acid in an inconspicuous spot for ~15m before rinsing -- no visible improvement. Should I try full strength oxalic acid or for longer?

Or should I first try another pass with 60 grit before going to 80? Go back to 36? Or something else entirely?

Your floor may have been covered with a poor man's rug. This is a square linoleum sheet with a pattern that looks like a carpet. Liquid spills around the edges can soak in because it takes longer to dry.
The stain may never sand out. Even the atmosphere will have a penetrating effect on bare wood over time and leave a stain compared to areas that have been sealed with a coat of finish or paint.

Thanks for the sanity check -- it helped me accept the condition of the floor as-is.

FWIW I did do another pass at 60 before moving up to 80 grit (etc) which did lessen the contrast of the splotches somewhat before sealing.

The kitchen is next, which is sure will be in worse condition -- er, I mean, is sure to have even more character.