Unwanted paint can alter the look of a smooth concrete surface. You can use several methods to leave a concrete wall looking as though there was never a trace of paint on it.
Clean the area of grime and debris using a clean cloth.
Scrape away as much of the paint as possible with a sharp object such as a razor blade or paint scraper.
Apply acetone or paint thinner to the area and continue to scrape until all the paint is gone.
Use a heat gun on the area to soften the paint.
Scrape away as much of the paint as possible, alternating between the heat gun and a sharp object such as a razor blade or a paint scraper.
Apply paint thinner or acetone to the remaining bits of paint and scrape them away as well.
A pressure washer may work well in removing paint from an outside concrete wall, but use caution as it may damage the surface.
Wear gloves when handling acetone or paint thinner. Use caution when handling sharp objects. Do not use a heat gun too close to a window; the heat could break the glass.