The key is one side or the end is fixed and you get the seal by torque on the free side, which forces a thread to thread seal. With the proper torque, no thread dope or tape is needed, except on Stainless Steel. Using NPS, both sides are free and if you are piping, like natural gas, your coupling must remain in tension set by proper torque. If that torque tension is ever released, you loose the thread to thread seal.
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Sorry, but I must disagree, James, with this and several other posts here. As correctly described in posts by Shadow and jibes138 whether the threads are parallel [NPS, BSPP] or tapered [NPT, BSPT] or even mixed assemblies of these there is clearance remaining between the thread roots and crests that provides a spiral leakage path thru the assembly. This is true no matter how hard you torque the assemblies together, no matter how long the engaged thread length you provide, and no matter what materials, stainless, bronze, plastic, etc. are used. That is why sealing gaskets or o-ring seals are provided for parallel threads and sealants like teflon tape and pipe seal compounds are provided for tapered threads and parallel threads.
The one exception to this are the NPTF "dryseal" taper pipe threads. These are much more expensive and designed for applications like fuel and steam piping where the piped fluid or the heat can dissolve or destroy sealants. The extra sharp crests and extra blunt roots of NPTF threads interfere and crush into each other forming a complete metal-to-metal seal on first assembly. Both fittings must be NPTF, and if ever disassembled must then be tape or compound sealed before reassembly or they'll leak too. Although expensive NPTF ball valves are available I don't believe thru-hulls or thru-hull flanges with this thread form are available. Given the ease and low cost of applying teflon tape and bedding compounds it makes more sense in recreational marine applications to simply use those.
Also, pipe dope/ Tef. tape are used to leak seal joints not to prevent them from rotating or loosening. Tapered threads tighten due to the taper. Parallel threads do not and unless "bottomed out" should be tightened against parallel threaded lock nuts or bonded seal washers or those Groco flanges described in this thread.
And, yes, that green verdigris you note on and around your thru hull is not Chia, it's evidence of leakage, hopefully previous and not ongoing, and likely future difficulty disassembling the parts.