Benzine used to be the fluid of choice, but it is highly flammable and also dissolves some of the printing ink used by the photogravure process. Carbon tetrachloride is also on the no-no list since it is considered hazardous to your health.

Trichloroethane (TCE) is a solvent used for photogravure printing ink and highlights thins, creases, holes, repairs and watermarks, plus drying quickly. It is non-flammable but is considered a mild carcinogen.

Clear and colorless TCE can be bought in paint stores and should be transferred to glass bottles immediately.

Lighter fluid can also be used as a watermark fluid, but it doesn't seem to be as effective as TCE in highlighting paper flaws. It is highly flammable, (no surprise in that statement), but the squirt top is handy in aiming streams of fluid at stamps.