Watermark detection

You will need watermark fluid or a watermark detector (machine). Some but not every watermark can be seen by holding a stamp to a strong light.

Preparation

 * You need to go in with some idea of what specific watermarks look like and their sizes. For example, when looking at US Washington-Franklins, be aware that watermark letters are seldom neatly centered on each stamp and can be upside down, backwards, or reversed. Having a stamp catalog, even an old one, can be very useful.
 * Sometimes, orientation of watermarks (upside down, reversed, backwards) can be significant, other times not. It can be useful in identifying issues, like in early Falkland Islands issues. Be aware that watermarks are typically illustrated showing the back of a stamp; confirm with your catalog or reference.
 * Be aware that, despite glowing claims for watermarking products, certain watermarks are hard to see like the spiral watermarks on British Commonwealth stamps of the 1970s. In some cases for classic stamps, you may see a portion of a papermaker's sheet margin watermark instead of the expected.

Using watermark fluid

 * You need to place a stamp face down against a black background, so you can also buy small black trays for watermarking. In a pinch, you can use the back of a black plastic approval card.
 * Only a few drops of watermark fluid are needed for a stamp. The fluid is volatile so you can waste a lot by using more.
 * This where your learning of possible watermarks comes in. Fluid will dry quickly so you need to do identification quickly. Just reapply fluid if needed.

Using watermark detectors (machines)

 * These use side lighting to show watermarks. They allow a longer viewing time than possible using watermark fluid, and there are no chemicals to breathe in. However, these can be ineffective if several hinges or hinge remnants are on the stamp back.
 * Adjust brightness as needed if you don't see a watermark immediately. You may have to turn the stamp 90° sometimes to get a proper view. If there is a colored light control, that may help also.

Tips

 * It can be useful to accumulate a group of stamps from similar-looking issues and watermark all in one session. Differences are then easier to identify, as in the cases of South African and Mexican watermarks.
 * Watermarking will also show thins, creases and repairs. In fluid, white spots appearing like paint are filled thins.



