Sealing TK820 unions

[HeatFarmer]

Any suggestions on the best way to seal these unions on a Thermovar tempering valve?

 

[Whitenuckler]

Any suggestions on the best way to seal these unions on a Thermovar tempering valve?View attachment 345139

We used to use the liquid teflon on some joints. I think gas guys use it.
Alsothis might be good:
Permatex® 59206 High-Temperature 592 Thread Sealant, 6-mL

 

[salecker]

Hemp is the best for sealing threaded joints
If you still have a leak after it is installed, it will seal it's self once the liquid gets to the hemp it will swell and seal.

 

[airlina]

Unions are a face to face mating surface seal , the threads are just for tigtening that seal. I have struggled a few times getting them to seal . First make sure the there are no dings or serious scratches on the mate surfaces, then alignment of the face to face union is critical and when I've had weeps a bit more torqueing has done the trick (to the point of needing a cheater extension on the wrenches. Only once I was not successful and had to replace the union with a new one that did the trick.

 

[Whitenuckler]

Unions are a face to face mating surface seal , the threads are just for tigtening that seal. I have struggled a few times getting them to seal . First make sure the there are no dings or serious scratches on the mate surfaces, then alignment of the face to face union is critical and when I've had weeps a bit more torqueing has done the trick (to the point of needing a cheater extension on the wrenches. Only once I was not successful and had to replace the union with a new one that did the trick.

I see a rubber washer in the picture...but it would be a metal-metal union only?