Trying to DIY refinish our hardwood floors. Got it sanded via a drum sander (60 grit), followed by two more passes with an orbital sander at 80 and 100 grit. Cleaned the dust and finished with an oil-based quick-drying finish.

Cleaning up the dust, my wife used a damp cloth. We suspect that left moisture in the wood, causing the problem (rookie mistake). But we tried sanding a spot, giving it 24 hours to dry, and refinishing -- yet it still has the same spots.

Two questions:
(A) Do you think our diagnosis is correct? Or could improper sanding somehow have caused these spots?

(B) If you do think it's moisture and 24 hours doesn't seem to resolve it -- any advice on what to do? Or how to best get it resolved?

picture here:
https://imgur.com/a/2FFflbi

You say you are refinishing the flooring. What finish was on the floor before you started?
It looks like there was some residual floor finish that was not removed.
To check on this, use a scraper to cut through to bare wood. Then wet with water. You ought to get a color close to your adjacent finished floor when it is wet. If old finish remained and you coated over it, you would get splotches. It looks like there was a water-based finish previously.
This happens frequently when refinishing the limed, "white stains". It is hard to know if more sanding is necessary to get all the finish.

We suspected water-based finish as the previous type because it was pretty worn down; the finish came with the house maybe nine years ago (and surely was originally applied well before that), and has been looking pretty poor lately. Add to that some furniture marking and a recent pet stain, and we decided it was time to tackle this.

We decided to tackle this with a buff sander (not sure if I have the name down right -- rectangular with a sanding pad) at 36, then 60-80-120 grit. It's a chore, but we struggled with the drum sander and I don't want to deal with that any more. Hopefully the 36 grit will help us take off that older coat, as it sounds like a good bet that's what we've been facing. Once we finish with the 36, I'll take a closer look at some of the 'splotching' areas and see if more is needed.

Very much appreciate the feedback. We don't mind putting in the work, but when things don't go to plan, it's sometimes hard to distinguish user error from other causes.

* correction of earlier post -- our original sanding was with a drum with 36-grit.

So, after a an initial pass with a pad/buff sander at 36 grit, we definitely see improvement in the white blotches. I'm inclined to your interpretation (Pete) that's it's a previous white finish of some sort... the floor was very blonde previously. I guess I never paid close attention... not exactly a floor connoisseur.

The thing that concerns me is that there are certain places (primarily edges/ends) where it seems the original stain really penetrated deeply. I'm guessing there's not much we can do about that? If you were faced with that type of problem, what would you do? Are there options?

Or is it (a) live with it or (b) chisel the boards out??? I admit I don't like either of those options.

Again, many thanks.

Floor sanders take a lot of the work out of refinishing. If there are spots that resist sanding you can use a sharp paint scraper to skim off the surface. It needs to be sharp to make a shaving, but shavings take less energy than hand sanding.
You should be able to see your progress by dampening with a wet sponge to see the color when finish is applied. Sand again after scraping to get the floor really flat.

Great Information.Well done