My local engieering company recommended a product called Curil made by a German company called elring, I believe.
It comes in two types - a hardening and a non-hardening formulation. He usually uses the hardening type, but I've opted for the non-hardening type - CurilK2.
I've tried hermatite and the blue stuff previously, but have always found that it make the gaskets stick the mating surfaces, so when I take bit off, some gasket stays on one side, and some on the other. AS well as that, it's a right palaver getting the surfaces coated evenly.
Since using this stuff, I haven't had any problems, and could even reuse the gasket, if no spare was available.
It's quite thin stuff, and I paint it on to the mating surfaces with a small paint brush, the size you'd use for doing plastic airplane models.
Touch wood - Haven't had any leaks from joints sealed with this stuff, so far,
John