What is the best method for cleaning copper.I have a building with a flashing strip on the roof,that is leaking green onto the concrete below.Not on the face of the building(brick),only on the base of bldg.,and the concrete sidewalk.There are painted alum window frames to deal with,below the source copper.Don't want to mar them.The green at the bottom of the wall is new,the problem is the flashing strip up top.Any ideas?Thanks.