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Shirley asked: How do I remove a stain on the hardwood floor from potassium hydroxide that spilled. During the night, some non-acid drain opener spilled on the hardwood floor. I saw the puddle and wiped up what I could with a damp cloth. It left a major dark stain. Can you help with suggestions?
Cleaning 
a dark stain from hardwood flooring will take a bit of work, but the results will be well worth the effort. The following solution involves several stages. If the stain comes up readily, you may not need all of the supplies listed below. On the other hand, if the stain is stubborn, you’re likely to need every one of them. For best results, follow the steps in order.
You Will Need:
- Fine grade 000 steel wool
- Mineral spirits
- White Vinegar
- Clean rag
- Bowl or small bucket
- Water
- Oxalic Acid
- Baking soda or ammonia
- Stain (to match the undamaged floor) plus supplies needed to re-stain
- Finish (to match the undamaged floor) plus supplies needed to re-finish
Steps to Remove the Stain:
- Before you begin, follow all label directions and make sure there is adequate ventilation in the area. Use fans if necessary to maintain air flow. Always read and heed all safety precautions, including those for skin and eye protection, and spot test a hidden area before you begin.
- First, rub the dark stained area with steel wool and mineral spirits.
- Next, wet the rag with white vinegar and lay it over the stain. Allow it to sit for several moments.
- If this lightened the stain, repeat as necessary. If it did not seem to work, move on to step 5.
- If you don’t notice an improvement, add oxalic acid to the stain. Do not get oxalic acid on the surrounding wood. It will seep in to the wood. Repeat as necessary.
- When the spot is gone, neutralize the acid with something alkaline, such as a mix of baking soda and water or ammonia and water.
- Wait for the spot to dry, then sand, stain and refinish to match the rest of the floor.
Additional Tips and Advice
- Be careful using the steel wool. Only go as deep as the finish or the stain. If you gouge the wood, there is no way to repair it.
- Do not allow the water-based cleaners to sit on the floor for an extended period of time as the moisture can damage the wood.
- Another method is to use a mixture of half water and half chlorine bleach, however, this must be done separately from the above methods as the bleach can react with the acids or with the ammonia and release toxic fumes.
- Always use caution when working with bleach as it is a hazardous chemical. See Wikipedia for more safety information.
Thanks for telling us about the vinegar for stubborn stains on wood floors. It really works! The only thing is, with a really stubborn stain, you have to leave the rag on for about a half hour each time. But a really great tip!
Vinegar and bleach make poison chlorine gas. This can kill you.
Your instructions don’t say to mix them, but don’t give any warnings either.
Yes, it will make poison. Just don’t use a gallon of each and remember that good old common sense works even better.
I am trying to find out how to remove black stain from hardwood floor from a flower pot that leaked.
Alison,
This is the article you need: How to Remove Black Marks Left by Urine on Hardwood Floors. The treatment is the same for water as for urine; its the moisture that causes the black stain. Good luck!
Oxalic acid is a powder. You don’t say how to prepare the mixture. Do I add water to make a paste?
Orlando,
The area should still be damp from the white vinegar, so the oxalic acid powder can just be sprinkled over the stain. Alternatively, yes, you could mix it with water to make a paste, which would give you more control over where it is applied to ensure it doesn’t get on the surrounding area, however, be sure to use as little water as possible to avoid further damaging the wood. You only need enough dampness for the powder to activate. Good luck!