When cleaning concrete, is there a time when paint cannot be removed?

When cleaning concrete, a wet rag alone cannot remove paint on a concrete floor or wall, when the paint has dried already. If the paint stain is small (droplets), a scraper can solve the problem, especially for oil-base paints, that form a thick film on the surface that make scraping easier. Paint thinner and a cloth, on the other hand, is the solution for paint that firmly adhered to the surface. Just lightly rub the thinner on the stain,  until the paint stain is transferred on the cloth with paint thinner solution.