Like many I have tried all the tricks trying to get a stock housing (both used and new) to seal including having a machine shop check and mill both surfaces.

Some attempts lasted longer than others but they all eventually leaked.

Thankfully the car is not a stock type vehicle so I spend a little extra $$ and bought this billet aluminum O ringed housing and have never had a leak issue since (4 years now)

I've had the housing off a few times and it has been leak free every time even with the same O ring and never use any sealant.

Somebody should make a stock looking T stat housing with an O ring.

https://www.summitracing.com/parts/bsp-90820