Pipe Dope [1]

The name commonly applied to the chemical compound used to ensure a tight seal on threaded joints between lengths of pipe, or lengths of pipe and valves or tanks, when they are assembled for use in a pump-and-tank system.

Pipe dopes are compounded of various chemical substances. The pipefitter spreads the paste-like dope completely over the threaded ends of a section of pipe before screwing it into place. Soon, the dope cures and provides a tight seal. Many pipe dopes do not set up hard. As a result, they permit the joint to be undone in the future.

It is important that the dope be compatible with the product stored.  A different compound should be used, for example, if the system is to handle alcohol blends instead of straight gasoline [2].