Personally, I would not polish the cable. If you have a cable friction problem, either something is wrong or you should lube the cable.
1. Get some motorcycle chain lube spray.
2. Remove the cable from the housing.
3. Remove housing from frame (or protect frame and brakes from excess lube, to come).
4. Using the extension tube attached to the spray nozzle, fill the housing with chain lube. Keep going until it drips out the far end.
5. Let the housing dry (a solvent needs to evaporate).
6. Reinstall the housing and cable.
The cable should move effortlessly through the housing after installation (check by pulling/pushing on cable by hand before installing in lever). If not, something is worng with the housing installation.
Very often there is a kink or poor routing or wrong housing length that causes flex when braking. This can be mistaken as cable flex. Sometimes, you can see the problem. If you apply the brake and the housing moves, something is wrong with the housing not the cable.
Be sure the ends of the housing are smooth and don't bind on the cable. After cutting to length, file them or grind them to be both smooth and perpendicular to the longitudinal axis.