Any suggestions to get my (new)Hardin thermostat housing to quit leaking? Have tried two new gaskets and still leaks, gasket sealer??

Not familiar with it, but if its a flat sealing face, you could try putting a sheet of sandpaper on a piece of plate glass or if you know someone with a granite surface plate and sand the gasket surface until you get uniform sanding marks all the way around the gasket surface.. It sounds like its warped or just not machined flat..

Thats what I would do, but if you don't feel like it, you could go the RTV route but then you have to remember to use that every time.

90MirageTrovare,

Can you confirm it's leaking at the mating surfaces?

Thanks. Brad.

Mine is difficult to fully seal, I had a bit of luck with it tightened down about 5-10° angled rather than straight on

Back in the 70's, our auto shop instructor showed us this technique on a certain rough but flat section of concrete driveway. LOL
We would color the sealing surface with magic marker, then "machine" the surface by holding it against the concrete with significant force and moving it in ~1ft circles until all the ink was gone. Good exercise! Worked well with cast iron housings. The Hardin may be alum??

If OP has a 1-wire temp sensor screwed into the housing, there needs to be an electrical bond between the thermostat housing and the intake manifold. The Merc gaskets use a copper(?) piece made into the gasket to achieve this connection. If sealant is used, it can block this connection. Good idea to verify housing is grounded with a meter afterwards if there's a sensor installed.