Because of the porous nature of concrete, the best way to remove stains is by absorbing the stain from the concrete material using trisodium phosphate for oil and grease or paint stripper for paint. Rust stains can also be removed, but requires that the surface layer of concrete be dissolved.
For oil and grease stains, mix 1 ounce of trisodium phosphate with a cup of water and enough absorbent material to create a thick paste. Absorbent material includes baby powder, talc and diatomaceous earth, Spread the paste over the stain, and allow the mixture to draw the oil or grease from the concrete, absorbing it into the absorbent material used in the paste. After the paste dries, sweep it up from the concrete, and then clear away any remaining debris using a nylon brush.
Oil and grease stains are difficult to remove, but oven cleaner or WD-40 may work on small blemishes. More heavy-duty stains need an absorbent solution to draw them out before scrubbing. Mix baby powder, cat litter or trisodium phosphate with water to make a paste, enough to completely cover the affected area in a thin layer. Allow the paste to dry before sweeping it away, scrub the area with a nylon brush and rinse it with clean water. The absorbent solution may need two or three applications to work completely, and some oil stains may leave a permanent mark.
Remove paint stains using the same process as oil and grease stains, substituting paint stripper for the trisodium phosphate and water mixture. Use a layer of stripper paste 1/4-inch to 1/2-inch thick, and wait about 20 minutes for the paste to loosen the paint. Scrape away the stripper, and wait for it to dry before disposing of it.
Other common stains include paint, epoxy and mildew. To remove household or spray paint, apply a paste of paint thinner and water to the area and let it sit for 10 to 20 minutes before scraping it away. For spilled epoxy, carefully burn the stain away with a blowtorch. To remove mold and mildew stains, use household bleach mixed with laundry detergent and water, scrub and rinse the area clean.
Rust is removed using muriatic acid. Add 1/2-cup of the acid to 2 cups of water in a large shallow plastic pan. Apply the acid to the concrete, and wait for it to stop bubbling. Scrub the area with a large handle nylon brush to remove the rust. Rinse after removal with lots of water to dilute any remaining acid. During all three removal processes, wear protective clothing and vent the room by opening any available doors and windows.