Do you use a gasket sealer on a gasket for water inlet?

I am going to assume that by water inlet, you mean thermostat housing. If the new gasket is a paper like material then by all means do so. Don't over do it, just a ultra thin coat on both sides to make it stick to the surface of the housing and /or engine. If it is paper like with a rubber o-ring inner flange than no. Final note, ensure that you purchase the correct tube of permatex (High Temperature) the back of the package will state (water pump/thermostat).

V/R

SSG Sadosky, USA

Armymech/Fordtech