Whether they occurred because your masking job was incomplete or because you simply forgot to mask in the first place, paint stains from overspray are no fun to remove from driveways or concrete walls, floors or walkways. You may have an easier time if the overspray is fresh , but that's seldom the case -- overspray dries quickly, and you probably want to finish the spray job before you start cleaning.
You may be able to scrub the paint off with soap and water or blast it with a power washer, but it's also possible that you'll need chemicals.
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Scrub with TSP
Most stain removal procedures begin with the gentlest cleaning methods and proceed to the most drastic, and in the case of paint overspray on concrete, scrubbing with soap and water is the gentlest method. There's a good chance it will work, too -- especially on sealed concrete floors and other smooth, non-porous surfaces -- if you use trisodium phosphate, a tough, grease-dissolving detergent that can do double-duty as a paint stripper.
Mix 1/2 cup of TSP in a bucket with 2 gallons of warm water. Put on rubber gloves and goggles, because TSP can burn your skin and damage your eyes.
Apply the solution liberally to the overspray and scrub with a fiber-bristle brush. The paint may come right off, but if it doesn't, spread more of the solution and leave it for 10 to 20 minutes to give it time to emulsify the paint. Scrub again.
Rinse the area well with clean water after using TSP. If you're removing paint from an outdoor surface, such as a driveway, rinse with a garden hose. To clean indoor surfaces, mop well with a wet mop.
Use a Power Washer
Paint can penetrate concrete surfaces fairly deeply, and scrubbing may not reach it -- but pressurized water from a power washer might. Try this option for outdoor surfaces, such as driveways, walkways and concrete walls, as well as for garage floors.
Strip the Paint with Chemicals
When all else fails, solvents that can dissolve the paint or -- ultimately -- a chemical stripper, may be your last resort.
Soak the paint with an appropriate solvent. Acetone and alcohol are two solvents that can soften latex paint. For alkyd and urethane paints, use lacquer thinner.
Scrub the paint off, using a fiber-bristle scrub brush. Avoid using a wire brush -- you'll probably end up scratching the concrete or removing some of it. Keep scrubbing, adding more solvent as needed, until the paint is gone, then let the solvent evaporate.
Apply a thick coating of concrete and masonry paint stripper to any paint you can't otherwise remove. Allow the stripper to remain on the surface for the recommended time period, then scrape it off with a paint scraper or a squeegee. Apply the stripper again, if needed.
Rinse the concrete well with water after using a chemical stripper, then let it dry.