Like polished glass! In the Bethwaite book they measure the turning force in pounds of rudder foils with 1200 grit finish, mirror polished, and they even chromed one (would never work in the real world as it flakes off too soon) and it was mid 80 pounds for 1200, mid 90's for mirror, and mid 120's for the chrome! On our Magic 25 which has a slender high aspect keel and rudder it was SO noticeable if there was the slightest film on the foils. The boat had gradually been broaching more and more, and the rudder didn't LOOK bad, but after spending a few hours mirror buffing it, it was an unbelievable difference! You could feel when the flow was about to shake off, and it would actually make this cracking sound as it stalled (way later into a broach) and you could quickly straighten the rudder and then steer again and the flow would reattach instantly.
With the dagger boards any vertical scratching from going up and down makes tiny channels that wick air down from the surface, killing the extra bit of pointing you need to be competitive upwind...