Shaving the run with a razor blade is the way to go. Shave it until you get it level with the surrounding area. Then block it.That way your taking the run down level,then sanding the area evenly. Another trick is if you get a run,wait until it is almost dry, and then recoat it. You will find that another coat over the dry run will not 'melt in' and will level it out a little and make it a LOT easier to sand out.It will leave a dry spot around the area but hey your sanding and buffing it now anyway.