when you're running without a gasket you want it as smooth as possible. mirror finish is a hyperbole but gives you an idea of the level of detail attainable.
here's a rechambered slant head I was working on before my camera quit on me. I still had some work to do on the squish band and one more stage of polishing to do, but you get the idea.
you never realize how bad the stock heads are until you attempt to rework one, tons of little bubbles in the aluminum and the finish on all of the surfaces are horrible.