I have never had a thermostat housing that did not leak !! No matter what I tried ! Some would seap a tiny bit, but there was never a totaly dry unit. That is until I tried this unit from Terry's Auto Lab :

http://www.terrysautolab.com/

Sure it is not cheap, but I DO NOT have a single leak on the car it is installed in !!! I really feel it is worth the $$ . . .

Good luck with yours . . .

Cheers

Mark