I have a great deal of trouble screwing on the pipe fitting (2" pipe) that connects my swimming pool filter to the pool heater. The pipes are composition and are to be hand tightened. Although the threads look good and have been cleaned, I need to use a large wrench to tighten the coupling with a great deal of effort. What must I do/need to make this coupling smooth and easy, before I strip the threads or break a pipe? Thanks for your help!