Safety Valve Leaking

Hi All
I have  stopped all the steam leaks apart from one on my Lykamobile series 2 . I cant stop the leak on the safety valve its leaking from the thread, its as tight as i can get it but still leaks if i can fix this then i can get the boiler inspector to pay a visit.
i have tried Steam seal, ptfe tape, permatex, and locktight nothing seems to work.
would be interested to hear from any one thats had the same problem or any help would be much appreciated.
Ray.

I have had great success with a product called X-pando. It is a dry powder that you mix with a bit of water to make a paste that you coat the threads. When it dries after assembly (over night) it expands and hopefully fills in all the nooks and crannies. It is for steam fittings.