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kitana at DIYFishKeepers says,
I first learned that the perfect plastic thread wasn't invented yet since it came out of molds and as always imperfect, thus allowing leakages to happen. Then, I learned that tightening the fittings up was the best way to break them because the male fittings were kinda wedged and would crack open the female fitting when tightened too much. I learned that Teflon alone was seldom sufficient to prevent leaks, that plumber's dope wasn't suitable for aquarium and wasn't very efficient for plastic pipe either.
Standard threaded fittings for PVC and most steel pipes are very slightly tapered.the diameter of the male threads increases slightly at the back of the threaded portion, while the diameter of the female threads decreases further inside the fitting.
Note: Steel pipes come with both NPT (National pipe tape) and NPS (National pipe straight) threads.
Jimbo at terrylove.com says,
Pipe dope and/or tape make plastic pipe too slippery. You should thread together hand tight...then at most another 1/2 turn.
Making Leak-Free Threaded Connections at IrrigationTutorials.com says,
Do not use “pipe dope” (liquid or paste type sealers) on irrigation systems!
It will get transmitted to drip emitters, and/or sprinklers and clog them.
When joining male and female threaded fittings, put a nice thick layer of PTFE thread seal tape on the male threads before you screw them into the fittings. Pull the tape tight onto the male thread so that the tape molds into the threads. Wrap it in the direction of the threads so it doesn’t unwind off when you screw the fitting on. (If you are looking at the end of the male fitting that would be clockwise.)
How much tape to use? The old standard was “3 wraps”. However now they are selling low-cost PTFE tape that is thinner and requires more wraps. When you have enough tape on the male threads the shape of the threads will be just barely visible through the tape.
Tighten it by hand (for a person with normal strength) and then add 1 full turn with a wrench. (Others say 1-2 turns)
The "Do's and Don'ts" of Assembling Threaded Plastic Fittings | Lasco Fittings
Teflon paste and pipe dope, just like Teflon tape, make threaded joints slippery. Their use on PVC fittings can be an invitation to over-torque.
When working with threaded plastic fittings do use a proper sealant. The right sealant for threaded joints is non-hardening, compatible with plastic and doesn't add slipperiness.
What thread sealant to use with PVC fittings! | Larry Workman LinkedIn
When assembling threaded PVC fittings, a sealing compound that is non-hardening is the best.
Many brands of pipe sealants contain oils, solvents or carriers that can damage PVC. A proper sealant must be approved by the manufacturer to be harmless to the joint materials.