Essentially watermark fluid is Petroleum Ether (Benzene), a dangerous and toxic chemical, which has been nuetralized rendering it safe and harmless. There are two ways of obtaining Benzene, one is by getting the 'Safe' version from a stamp supply store. The other is to purchase the lighter fluid Ronosol, sold at most markets. The latter is the unsafe version, you should not let it touch your skin or breath the fumes, I would stay away from using it. Most old stamp hands will tell you to use the cheaper and slower drying Ronosol, but before doing so, read the toxicology on this chemical.

USING WATERMARK FLUID

Place the stamp, face down in a black or very dark tray, ideally a small tray so you can use less of that expensive watermark fluid (a small bottle costs $12), or if you plan to watermark a lot of stamps invest in an inexpensive ($2) watermark tray. After laying the stamp face down in the tray, pour enough fluid to wet all the stamp. Watch closely as soon as you do this. If the stamp has a watermark the fluid will expose it, it may look like this:

WHAT TO LOOK FOR

Once the stamp has become soaked in the watermark fluid, the watermark will appear. It will be either a single lined letter (the first image), or a double lined letter, (the second image). As you can see from the arrangement on the sheets of stamps, you may see only a partial(s) letter.

LOOKING FOR HIDDEN FAULTS WITH WATERMARK FLUID

If watermark fluid exposes a dark line(s) across all or part of it, then the stamp has a crease or a tear, if there is a dark spot, no matter how small, this is a thin (see below for a explanation of what a 'thin' is). Even the smallest tear or thin will seriously effect the value of a stamp.