What must be done to piping threads before they are joined?

Cleaning piping threads before they are joined is essential to ensure a proper seal and contact between the threads. Contaminants such as dirt, oil, or old sealant can prevent threads from mating correctly, leading to leaks and potential failures in the piping system.

By thoroughly cleaning the threads, you remove any debris that could compromise the integrity of the joint. This step also helps in any further treatment, like applying pipe thread sealant or tape, which would also require clean surfaces to adhere effectively.

While painting, polishing, or rustproofing threads may have their own specific applications, they do not directly contribute to the immediate need for a secure and leak-free connection in the context of joining piping threads. Thus, cleaning is the most critical action to take before making these connections.