I like the Blue Block also. When messing with gas or boiler stuff that's all I let my guys use. It's pretty gummy, try not to get it on you, you'll look like you got in a fight with a smurf. With black iron with pipe dope if you have a leak a lot of times it will stop when the system is heated up (things expand), if you have teflon on it you either have to redo the joint or tighten it more, either way you have to take everything apart to the joint that is leaking. That's my 2 cents worth after 30 years.