Public urination has gotten so bad in San Francisco that the city has painted nine walls with a repellant paint that makes pee spray back on the offender. It’s the latest effort to address a chronic problem in a city where the public works director calls himself Mr. Clean: Walls are coated with a clear, liquid repellant material that goes on much like paint. Hit with urine, it splashes back on a person’s shoes and pants. Mohammed Nuru, director of San Francisco’s public works department, says offenders will need to make the mistake only once to get the idea. “If you have to go,” he said, “go in the right place.” Nuru got the idea from Germany, where walls in Hamburg’s St. Pauli quarter are painted with the material to encourage late-night beer drinkers to find a bathroom rather than an alleyway.