This is what steelconduit.org says about it
"14.Factory cut threads have corrosion protection applied at the factory. Field cut threads are required to be coated "with an approved electrically conductive, corrosion-resistant compound "where corrosion protection is necessary " (see NEC® 2002 300.6 (A). We recommend that all field cut threads be protected against corrosion where they will be installed in wet or outdoor locations. The thread surface should be protected with conductive rust resistant coating. Zinc-rich paint is a typical coating, but there are other conductive coatings that can be used.
Field threads should be cut one thread short. This will insure a good connection and allow the entire thread surface to be inside the coupling."
