If you give away or sell prints is it customary to sign the print, or the back of the print? If so, what is the best instrument to use for signing B&W paper? My first thought was a grease/china pencil, but I thought that might smear/rub off. My second thought was a felt tip permanent marker, but then I thought it may seep through. Other felt tip markers may rub off. Then I thought paint pen. Any suggestions?