Bethwaite in "high performance sailing" writes about a test( page 233) he did with 2 rudder blades with NACA 0009 sections. they were prepared identically to 1200 grit finish then one was buffed to mirror. The dull (sanded) one apparently generated 23% less cross flow lift force. Apparently this tallies with expectations from theory.
This then rather suggests that polished is better
can anyone explain how the drag will be lesser over a sanded rougher surface than over a polished one?
I can believe that there is a permissable level of roughness depending on the boundary layer thickness, which would mean once sanded to a certain smoothness there isnt much benefit in going smoother(ie polished) but to say that polished is worse than sanded? i havnt seen any evidence of this.