I set up a sheet of glass that was extremely dirty and swabbed on some water based house paint.Just before it dried I used a damp cloth to wipe a window, then used a latex glove & a cloth to finger paint and swab the outside frame. Put a light to the side just behind, set up the camera and shot. Processed in CS5 dodge, burn, curves etc until happy. Copy of a much better one of these on //500px.com/photo/755651