Best iron-thread sealant

I still use boss-white/hawk-white or similar for such joints. I strip down a short piece of "hairy" string - usually garden stuff as a hemp substitute and use that as a filler with the boss-white. I expect there may be more modern solutions if you have to buy something especially but the old ones still work if you have a tin of BW in the shed.

I use PTFE tape - it's "set" to start with, so the water pressure can't blow through it before it "sets". The thinner white spooled tape can give you more control over the thickness you use to end up with it sealing in the right position.

You will want a 1/4" taper female coupling, unless the male part has a sealing flange (in which case you don't want a sealant at all, but a sealing flange will be hard to orient in the right direction, if the gauge doesn't have a swivel joint built-in).

It has to be rated for potable water - which sadly those aren't :(

or similar for such joints.

I tried Boss Green but it is truly horrible. You can get PTFE string to replace the use of hemp, but I'd prefer a modern "loctite" type liquid.

Just wondered if anyone had a favourite :)

Ah - that's what I did last time...

That's a conumdrum... The last time I had one of these gauges, it looked like it was designed to bottom out and take a fibre washer or O ring - but the thread simply was not long enough for the marrying female part (which was parallel - essentially it was a Caleffi PRV with inbuilt gauge so all parts came in the same box). I decided to get a new PRV so I can have the far end rebuilt before I need to break the current water connection. I decided this time to use a gaugeless valve and have a gauge on an isolator valve - so I do have some say over the female part.

I do get the chance to swivel the whole tiny stub pipe on the isolator compression joint :)

I have used the Loctite yellow gunk before but it makes disassembly rather hard as it sets like glue.

I used PTFE paste very successfully on a steel pipe air line, working at