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Oil stains are challenging to remove, especially if they are not noticed until the oil dries and seeps into the surface. When oil spills occur on concrete, such as from a car or lawnmower, the porous surface of the concrete absorbs the oil, making it even more of a challenge. The key to completely removing the oil stain is to create an acid solution that dissolves the oil even in the deepest of crevices.
Fill a bucket with six parts water and one part trisodium phosphate (TSP). Stir the mixture with a paint stirrer until it forms a thin paste. Spread or pour the solution onto the oil-stained concrete.
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Check the paste periodically until it is completely dry and begins to flake. Scrub the surface of the stain with a stiff-bristle brush to dislodge the paste, and then rinse the concrete with clean water from a garden hose.
Rinse out the original bucket so that no residue remains. Pour sodium hydroxide into the bucket and add water until it forms a 5-percent solution. In some hardware stores, you can find sodium hydroxide already premixed to the proper dilution.
Pour the sodium hydroxide solution onto the oil stain and let it dry for 24 hours. Scrub the stain with the stiff-bristle brush and then rinse it well with a garden hose.
Examine the concrete to see if the oil stain is gone. If any oil residue remains, repeat the application of 5-percent sodium hydroxide to remove it.
Apply mineral spirits on the previously stained area until it is saturated. This will neutralize the acid residue. Allow the mineral spirits to dry and remain on the concrete for one hour. Rinse the surface a final time while scrubbing it with the brush.
Acid solutions are readily available at any hardware or home improvement store.
Always wear safety glasses and rubber gloves when working with the acids, since they can cause injury to the skin and eyes.