the blue gaskest sealer for water systems. i usually wait for a few hours after application to run the engine, so it has time to seal/harden enough not to swim around in the coolant lines. and plug the NEW THERMOSTAT... dont ask why i know that....
the name of it is, Ultra Blue Sealer.
Advice... dont buy the big tube. if it is months between usage, the rest/balance of te tube will or better said, might harden in the waiting period. so, i end up waisting most of my purchase... if your doing lots of sealing, then the bigger tube. if just the thermostat housing, small tube.
Simple RTV is what I always use. There are some generics and a high temp version. I have used the high temp stuff to seal a chamber attached to an exhaust manifold, no problem. Good stuff.
Probably half the price of something called "Blue gasket sealer" and in the end it is the same thing. The tube I have has been around for 10 years plus. Looks like the day I bought it. I'm sure the Blue stuff is better know matter how many times you need to buy it.