How can I remove black discolouration/black stains from hardwood floor

I use wood bleach, oxyalic acid, mixed in a glass jar using a chip brush to apply on the spots. You may have to reapply frequently as it stops working when dry, and some areas that might be darker and deeper in the wood will require more applications.

Thanks for replying, Sharon. Could you possibly share oxalic acid and wood bleach brands you use ? And do you use a chip brush instead of a paint brush because the mixed oxalic acid and wood bleach would melt the bristles of a paint brush ?

Try hydrogen peroxide. Whatever bleaching agent you use, apply sparingly as water products and wood are not very good together.

Here's one way

https://www.hometalk.com/diy/clean/floors/pet-stains-on-hardwood-floors-44360112

I would try hydrogen peroxide first. That sounds less damaging to the wood than bleach. But just like teeth whitener that uses hydrogen peroxide, sounds like would need a few coats.

You can rent an orbital sander for a day or a weekend. It really should only take a day to do. It will go fast.

Could you have a pro come and look and consult? It would be such a shame to try some recipe that makes a mess of them!

Thanks, everyone, for your suggestions. I will try the hydrogen peroxide - on a board in an inconspicuous area first.

Hi! Here's some advice on the hydrogen peroxide method:

https://floortechie.com/black-water-stains-on-hardwood-floors/

Are they actual stains or is it the wood itself? It could be the nature of the wood itself and in that case, you'd likely be fighting a pointless battle. Do you know what kind of wood the floor is? If the more you sand, the worse they get, it might be the wood.

You can make use of some hydrogen peroxide then soak a cloth in it and dab it onto the areas which there are discolorations. Allow it to dry overnight. Repeat as needed.

Hello,

Try anyone of the methods you have already been given in the most out of the way place and see if it works. Theses marks are part of the natural wood and may never come out, but may be lightened.

These dark areas may be part of the natural wood's complexion, Do you have any idea what caused the dark stains if they are not natural? Sounds like you have sanded and sanded as much as you dare.

Here's a link to bleaching options:

https://www.hunker.com/12413070/how-to-bleach-hard-wood-floors

Home Depot and Amazon have oxalic acid, maybe Walmart.

Here's a post that might help https://floortechie.com/black-water-stains-on-hardwood-floors/#:~:text=Mix%20a%20cup%20of%20white,smell%20on%20your%20wood%20floor.

Yes, Wood Bleach or Oxalic Acid.

Fine Furniture restorers swear by oxalic acid.

Wood bleach is usually a two-part product and you mix batches as you need them.

Sav-o-grain is the brand I’ve used.

Thanks so much everybody for all your suggestions. I have taken them all on board and researched them too. And I have decided to go with a product called Liten-it: a high-strength, VOC-free wood bleach that purports to not damage the wood fibres and also will lighten all wood, as well as remove the black discolouration. (After which I will apply clear, matt water-based polyurethrane as a finish to protect.) It's a pricier solution than I had hoped for, but I am preparing my house for sale - and would like to floorboards to look as natural as possible. Thanks again for all the suggestions.