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Pipe dope is a compound that can act as a thread sealer or lubricant. It is mainly used when joining pipe fittings, which are designed to be capable of creating a positive seal on their own through the process of interference. Tiny imperfections in the material can result in a less than perfect seal, so pipe dope can be applied to fill in the gaps and make the joint water or air tight. The other main purpose of pipe dope is to keep fittings from seizing together so that they can be disassembled later on if necessary. Thread tape is a similar material that is applied in thin strips rather than painted on with a brush.
When pipe threads are tightened against each other, they tend to wedge in place and create a leak proof seal. Pipe threads are designed with a specific taper that can allow them to wedge together into a tight, interference fit. Thread sealants, such as pipe dope, can help fill in any nicks, worn threads, or various manufacturing imperfections, and is typically applied to the male end of a pipe. A thin strip of sealer may be brushed perpendicularly to the length of the threads, or wound around the circumference. When the female end is connected and tightened, the sealant is drawn in and among the threads where it is able to help facilitate a seal that is capable of withstanding some amount of pressure.
There are different varieties of pipe dope that are made with specific circumstances in mind. Each type of sealant typically has a pipe material it works best with in addition to an intended application. There are pipe dopes specifically designed for caustic environments, water, oil, air, and a variety of other uses. Sealants also exist that are designed for use with various kinds of plastic and metal piping. Using the incorrect pipe dope for an application can result in the sealant breaking down, which may lead to a leak.
Beyond the primary function of creating a tight seal, pipe dope can also provide a valuable lubricating function. After pipe threads have been mated together for a long period of time, they may become seized. This is particularly common with certain types of metal pipes, though it can also occur with common thermoplastic polymers. If a lubricant such as pipe dope or thread tape is applied during assembly, it will typically be significantly easier to split the pipes apart for any repair jobs at a later time.