This section presents our watermark detectors. Collectors know that some stamps have an invisible design printed in the paper called a watermark. The best and sometimes only way to let it appear is to treat the stamp with watermark detector fluid. This liquid is safe for all stamps, even photogravure, lithographed or engraved. It will not harm the gum on your stamp, and the ink will not run. We sell watermark detector fluid in two convenient sizes.