Reference: J. Anal. Chem. 56 (2001) 542-545. "The maximum smoothness with the minimum gold loss was provided using electrochemical polishing in a solution of 80 g/L thiourea, 2.5 g/L ammonium thiocyanate, 14 g/L Sulfuric acid and 8.8 g/L lactic acid, current density 6 A/dm2, 18 degrees celsius for 2 minutes."

Gold is quite soft and mechanical polishing leaves particles embedded into the surface.

I hope it helps.

http://link.springer.com/article/10.1023%2FA%3A1016680727301#page-1