RA = Roughness Average

Generally speaking, you should be able to see your reflection in a machine surface that can handle MLS gaskets. Without sophisticated and modern equipment, most machine shops can't get near that level of smoothness..... particularly on both heads and block deck surfaces. But, in addition to the polished surface, waviness or undulations need to be considered.... and these are very hard to see with only your eyeballs.

Larry Carley on MLS Gasket Surfaces

Brendan, on a relatively new engine that was factory machined for MLS gaskets, you can usually replace the MLS gaskets. The problem comes into play when you get the head or deck cut at a machine shop that can't duplicate the factory finish.

Frank