Polishing parts of the trigger meganisem is done to minimize friction in order to smooth the trigger action. At the underside of the triggerbar you find two grooves that rides over a spring mounted in the frame. These are roughly made, at least at my CZ75SA. After a trigger job I could really feel these roughness. After polishing these I got a excellent smooth pre-travel!
Polish these grooves with a small diamond file or other small round file, than use waterproof in various grit (600,800,1000) sandpaper and last a polish machine (or dremel) to a shiny finish.
Part of this earlier post from me. (Posted as CZ75SA but that's me too!)
Jeroen
