Suggestions to seal a new thermostat switch in manifold

[OUbesto]

Guys, with the help from this forum I just installed a new Champion aluminum radiator with dual electric fans. I installed the thermal switch (185/170) that I purchased in a relay kit for the fans. The instructions recommended not to use any thread sealant because it may interfere with making a proper and consistent ground. I also read somewhere on the forum not to crank down too hard. Unfortunately I still get a small amount of coolant leaking around the threads. Anyone have suggestions to stop the leaking?