The thermostat and the cover had some build up. Good thing I was careful removing the gasket, as the new thermostat did not come with a new gasket.

I started cleaning up the thermostat seat area by just scraping, but I could see that it would not result in a smooth seat.

I switched to industrial vinegar (10%) and later to muriatic acid. That cleaned things up in a hurry. I actually soaked the old thermostat clean and kept it for a backup. A car thermostat, which doesn't get any build up, is good for decades. Why not an outboard thermostat with the build up dissolved periodically?

Soundproofing added to the inside of my F50 cowling.