Can't get NPT connection to not leak

Question

I am trying to connect this spigot (3/4 in. FIP Inlet):
https://www.amazon.com/gp/product/B00MA9F8RE/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
to this elbow (3/4" NPT Male):
https://www.amazon.com/gp/product/B004XUKSF0/ref=ppx_yo_dt_b_asin_title_o00_s01?ie=UTF8&psc=1
If I understand correctly, FIP and NPT are directly compatible - right?
Even with paste thread sealant (which I believe you are supposed to use for this kind of connection), and tightening it wrench tight, it still leaks. Not spraying, or even dripping, but just pooling up water enough to drip one drip ever several seconds. This seems wrong. I noticed that when I unscrew the threads, they are really clean (i.e. the paste sealant seems to have been "pushed" out entirely when I connected the two).
Am I doing something wrong here?

d.george · Answer

Try this; use an RTV silicone on the threads instead of joint compound or teflon tape. Put it together and wait a couple days to turn the water back on. That should work and stop the leak but if it doesn't work get some Expando, a gray powder that you mix with water, mix it to a thick paste and apply as the pipe dope. When it dries, it expands and will seal just about anything. Just a note, I have had better success with the RTV than with Expando.

Answered by d.george on July 15, 2021

d.george · Answer

I put H B Smith 350, 450, and 650 screw nipple boilers together with RTV for many years as did a couple contractors I worked with. These boilers almost always had nipple thread leaks. When I or the contractors used RTV we never had a leaking nipple.
my 2 cents

Can't get NPT connection to not leak

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