A dark spot left where the mouthwash absorbed is a true marble stain.
This is different than the lighter or chalky dull spots sometimes confused as stains but are actually etch marks from acidic or chemical corrosion of the marble surface.
I mention this because depending on the type of substance you can get a stain, an etch mark, or both in the same spot. This could happen with a mouthwash stain.
Repair or removal is completely different for each type.
And you may not see the etch mark until the stain is removed. More on that below.
To remove a mouthwash stain from marble you'll need to make and apply a poultice... possibly several times, but you should be able to remove it.
More than likely, the mouthwash is acidic enough that it has also etched or corroded the surface destroying the polish, so you'll have to re-polish the surface after removing the stain.
Make A Mouthwash Stain Removal Poultice
Making a stain-removal poultice is simple and you probably have most ingredients already. Note that the exact mix or ingredients will change depending on the type of liquid or substance (oil, dye, organic, etc.) caused the stain.
To remove the mouthwash marble stain we're going to use hydrogen peroxide in the poultice, but that would change for a stain from some other liquid.
1. Buy 12% Hydrogen Peroxide (the 3% drug store kind is too weak) and some corn starch or baking soda. Even flour can work.
2. Mix the corn starch and hydrogen peroxide into a paste like peanut butter. This is your "poultice."
3. Apply the poultice to the stain 1/2 thick with a 1/2 border.
4. Cover the poultice with plastic wrap, seal the edges with blue painters tape and let it sit 48 hours. Masking tape can be used but it may leave a sticky residue.
5. Remove the plastic wrap, but don't touch the poultice. Let it dry another day until it is hard. The hydrogen peroxide breaks down the stain and as the powder dries out it sucks it all up.
6. Scrap off the poultice with plastic or wood spatula and clean with hot water or a marble cleaning spray which is best for regular cleaning.
7. Repeat the above steps (if necessary) until the mouthwash stain is gone.
Deep or old marble stains may need several applications of a poultice.
Repolishing a Mouthwash Etch Mark
If after removing the mouthwash stain you notice a dull spot on the marble countertop, then the mouthwash also etched the surface.
Remember, this spot is different than the mouthwash stain.
The procedure to remove an etch mark with this product is simple as well (basically just apply, wet, rub for 10-20 seconds, wipe off immediately) and will restore the shine and color of the marble countertop.
If you don't get satisfactory results from this procedure, then the damage is severe and will require professional help. But I think you'll be able to get the job done yourself on this one as severe etching is rare.
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