When doing a ductless AC system do you put this on the inside and outside of the flair or only on the threads?

This stuff is great. You don't put it on the thread, it goes on the flair. It's not like lock tite it's a flair tight. I'm a diu girl and this is great stuff. I used it a year ago and I still have great ac. Before using it I could not hold a vacuum on my system.

Will this work with propane gas lines

Having not used it, can't say 100%. But since R290 is purified propane, and this says it's for all refrigerants, I don't see why not. Though there may be additives in propane meant for heating that could possibly interact with it. Certainly propane for heating is not as pure as the refrigeration stuff. As someone else mentioned, asking the manufacturer is probably a good idea before trying.

can this be used to seal service valve caps?

I don't believe I would use it for that purpose. If it gets into the valve it would probably keep the valve from working properly. I generally use brass caps with o-rings to seal service valve ports.

ASIN	B008HOSQQQ
Best Sellers Rank	#65,173 in Tools & Home Improvement ( See Top 100 in Tools & Home Improvement ) #369 in Hardware Sealers
Item model number	RT201B
Manufacturer	Refrigeration Technologies
Product Dimensions	2.54 x 2.54 x 1.27 cm; 22.68 g

RT201B Nylog Gasket/Thread Sealant, 30 ml (1 Pack)

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