The first thing about those tapered threads is that they have to be within tolerance. Simple practice in a pipe fabrication shop is to adjust the threading dies so that a sample thread run into a fitting becomes too tight to continue by hand after no more then three turns.
If you are using teflon tape pipe sealant, or any of the brush on sealants, and after pulling the threaded pipe end and the fitting up tight you are still leaking it is because the threading dies are cutting too deep defeating the purpose of the taper because even when you run up to the end of the thread you still are not getting the appropriate squeeze which is the entire purpose of tapered thread cutting. Pipe dope, e.g., teflon tape or or brush on, are intended to seal irregularities in the surface of the metals but the main sealing effect is as a result of the taper squeeze.
Any other solution simply asks for more trouble.
Do it right. Go back and check the adjustment on the threading dies so that you cannot, by hand, run the threaded pipe into the fitting (Or coupling) by hand more than three turns before it is needful to utilize pipe wrenches.
"Never continue in a job you don't enjoy. If you're happy in what you're doing, you'll like yourself, you'll have inner peace. And if you have that, along with physical health, you will have had more success than you could possibly have imagined." -- Johnny Carson