Thread sealing.

Recently removed 1.5" gate valves mated to skin fittings with 'foliac' and while seal seemed good it was very difficult to remove even with heat. I am considering using 'bosswhite' or similar when re-fitting ball valves but had been thinking of seeking advice.
 
PTFE is OK for parallel threads, and works fine on (all my) underwater fittings.

Do the usual 2-3 wraps and do a trial assembly to see if it is reasonably tight when you reach the orientation you desire.

If not, adjust the number of wraps to acheive the desired effect.
 
if the threads are a bit slack get some plumbers hemp and boss white.

Smear the threads with boss white then tightly wind strands of the hemp into the roots of the threads . Start by almost filling the threads on the male part with hemp strands.

wind on clockwise so screwing the parts together tightens the strands in the threads.

Bit messy but very watertight and will not become loose.

if the threads are very slack you can get a backing nut (Plumbers/pipe suppliers) to wind back against the face of the fitting with bosswhite and hemp forming a seal in between.
 
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