There is such thing as too fine a finnish.
You need some roughness so the parts can geta good grip on the gaskets.
I have never gone finer that 240.
Then I lightly grease the parts with some copper antiseize.
Funny thing about that stuff.....
When it gets hot it carbonizes and bakes hard. Its not near as good as proper gasket sealer but it fills tiny voids and cooks there on an engine, In some cases it actualy becomes siezing compound if you use it on things like exhaust manifold nuts.