Most collectors are familiar with the use of watermark fluid to detect watermarks on early US stamps. Did you know there is another important use for watermark fluid?
It is also helpful in the examination of stamps when trying to detect thins. A watermark is nothing more than a deliberate thinning of the paper in some particular design for security purposes. Technically, watermarks are very shallow thins in the paper.
If you suspect a stamp might have a thin, dip it in watermark fluid. Thins, even very minor ones, show through as a dark spot on the stamp.
Sometimes thins are filled in to hide them. Again using watermark fluid, the paper of the stamp should look very uniform. If there is an odd shaped patch that is darker or lighter in color than the surrounding paper, there is a very good chance that the stamp was thinned at some point in time. Someone tried to repair the thin by filling it in with other wood pulp.
Some filled in thins are done so professionally, itís very difficult to spot them with just the naked eye. However, matching paper characteristics is virtually impossible to do. Therefore, filled in thins will differ in appearance when submersed in watermark fluid.