Antifreeze leak on header flange

I like teflon tape works as does silicone. I would get some high temp silicone that will stand up to the heat from the manifolds. You should probably drain the coolant and dry the hole(s) if you use silicone. When you unscrew the bolt or stud it will pee out coolant for quite a while. You might be able to unscrew, dry off the bolt, wrap with teflon, and screw it in without loosing more than a gallon.