Over the last few years we've had several customers call with questions regarding the effects of oil-based pipe dope compounds on the threads of Hunter sprinklers. In case you were unaware, the two are not meant to go together and the end result is undesirable.
The body, riser, and inlet threads on Hunter Professional and Institutional series sprinklers as well as most other brands of sprinklers are manufactured with a durable plastic material called ABS (this material is also sold under a number of brand names other than ABS).
When pipe dope comes into contact with ABS plastic, certain oil based chemicals in the pipe dope attack the plastic and cause it to become very weak and brittle. Within days the weakened threads around the sprinkler inlet may get stress cracks and can leak profusely. Pipe dope is intended to be used as a sealant on metal-to-metal pipe threads only and thus should never be used as a sealant on sprinkler inlet threads manufactured with ABS. For this reason, we print "Use No Pipe Dope" on the body of Hunter sprinklers. Standard Teflon tape or plastic compatible Teflon sealant around the riser or swing joint threads is all that is required to prevent leaks around the inlet threads of Hunter sprinklers. The Teflon also provides a long-lasting lubricant should it become necessary to remove the sprinkler for servicing.
Many other plastic components are negatively affected by pipe dope, thus we recommend that the sealant standard should be Teflon tape on the threads of any components that are metal-to-plastic or plastic-to-plastic.