As long as your thermostat has a rubber seal that came with it, It shouldn't need any other sealant. Adding other sealant can actually make the situation worse. Just place the rubber seal on the housing and tighten the screws down evenly and slowly and it should all be good.

 

Thank You so much for the quick response, I thought so but would rather ask first

 

I've never coated the threads on the retainer bolts or used sealant on the thermostat housing, no leaks !
Don't forget to park your HHR nose high and let any air burp out the filler cap!

 

Thank you. I have the front end up on ramps