Why are there dark stains left on my oak floor after sanding them?

Question

Should we keep sanding them more, with stronger sandpaper, or will this damage the floor?. Will oak always have dark spots left?. I was trying to get them as light as possible. Thanks

Answer 1

I'd like to know about the wood bleach also!!!!  I thought about trying peroxide on mine.  Please let me know if you find out!!!!!!!

Answer 2

I suggest a good wood bleach treatment until the stained areas are close to the rest. Sanding will just keep removing good wood.

EDIT: You can find the wood bleach at any good lumber yard and even the local Home Hardware has it. Don't use reg. bleach.

Answer 3

Those could be historical spills. Oil, chemicals, soap... Of the past.

Answer 4

First step is to sand a little more, but not so deeply that you reach the tongues and grooves.

Very dark stains in a wood floor are often the result of pet urine.  If that is the case then you probably will smell it, and you should be warned that pet stains are very difficult to remove.  Many other stains are caused by dirty water that leaked from a planter.

Anyway, if you want to try bleaching these spots and have already sanded the old finish off the flooring, be aware of the fact that bleaching may cause the surrounding unstained wood to lighten up as well, creating a donut halo effect.  To prevent the halo, you might mask off the first 6 inches around the stained area or apply a bit of new finish there (that may be sanded off).  Although you might try bleaching with peroxide, I would advise against using Clorox, because it damages the wood.  

I have used a product named DeksWood (manufactured by Flood, Inc. that contains oxalylic acid) with mixed results.  The product is milder than Clorox, and is actually intended for renewing grayed-out redwood decks and cedar fences.  On a hardwood floor, pour a 6 inch puddle of Dekswood onto the stained area, then scrub with a scrub brush for a couple minutes, let it sit for 15 minutes, then wipe off with a damp rag.  Wait 30 minutes, then repeat over and over.  

Any bleach will raise the wood grain and require a light resanding with #80 grit or finer.  Some stains will never entirely come out and you either live with them or patch in new boards, so consider any bleaching efforts to be an experiment.  If your heart isn't set on having a natural colored clear finished floor, you can reduce the contrast between a stained area and the rest of the floor by applying a coat of dark stain to the entire floor before applying coats of finish.