Today someone told me that the application of PVC primer prior to the application of PVC cement is not necessary since the pipe will not be used for liquid transport. Is this true?The PVC cement instructions call for the application of primer, but provides no amplification on such conditions where it is unnecessary.
The reason primer is required is that it breaks down the poly-vinyl chloride polymer chains (softens the material). The glue must be applied immediately after the primer, and very importantly before it dries. In my experience this is very quick, a couple seconds at most. The bond that is formed between the PVC surfaces is not one of simple adhesion, but a “solvent weld,” where the PVC surfaces bind with each other. This is the reason why various building codes require primer on PVC plumbing.
PVC glue without primer is therefore much less strong than a primered joint. Likewise, a solvent weld is not formed if the glue is applied after the primer is allowed to dry. If using the pipe for liquid transport, these scenarios are very prone to leaks and breaks, especially under pressure.
