Thread Dope compound is lubricant, thread sealing compound, and an anaerobic chemical sealant that is used to make a pipe thread joint leak proof and pressure tight.
Although common pipe threads are tapered and therefore will achieve a proper fit during proper assembly, machining and finishing variances usually result in a fit that does not result in 100 percent contact between the mating components. The application of thread dope prior to assembly will fill the minute voids between the threads, thus making the joint pressure tight.
Thread dope also acts as a lubricant and helps prevent seizing of the mating parts, which can later cause difficulty during disassembly.
Various types of thread dope formulations exist. The appropriate type being determined by the application, e.g., pneumatic, hydraulic, caustic, etc. Improper selection of the type of thread dope may result in leakage despite best assembly practices.
Petroleum-based thread dope is not intended for use on threaded PVC, CPVC or ABS pipe and fittings since it will deteriorate the plastic.