I used to use a much courser finish up until recently, that was provided by a multi-carbide cutter head on my machine. 16 carbides actually. Always worked great for anything, steel shim, or regular composition(Fel-pro).
Last few years, all the Ford geeks W/aluminum heads/blowers, and cometic gaskets, wanted that real fine,(looks like a "prism", mirror shine in the light), finish for their heads, so I switched to a single CBN and wooped up the RPM with a real slow traverse.
Looks real pretty, and it's what the cometic manu's specified as rms finish. Dunno, I better check into it closer when I got more time. My bum still hurts from the cost of retro-fitting my machine !
We always used to spray "copper-Kote" on the old steel shim stuff for added integrity, seems to me, that "fine finish" may "hydraulic" when torqued using copper spray on the regular old shim. The cometics are a 3 pc with the steel in the middle sandwiched by a impregnable sealing surface, they go on dry.
Dunno, I just do what they/people want for machine pkg's for shops. My preference, I'd still use the old finish for everything I'm screwing together for myself, it's proven, and has good bite !