Thermostat housing leaks

Try sliding it over a piece of sandpaper on a piece of glass to see if there's any high spots. That's more common on the stamped steel housings. If yours is cast, look closely for hairline cracks. Of course, be sure all the old gasket material has been cleaned off. I like to use a thin coating of silicone gasket sealer on both sides of the paper gasket in case there's sanding scratches or pits that won't otherwise seal well.