There are 3 different sizes of alumina powder for polishing the GCE surface namely 1.0, 0.3, and 0.05 microns.
Alumina with a particle size of 0.05 micron is commonly used. After polishing the electrode surface with 0.05 micron slurry, if some scratches or residues are still found on the electrode surface, continue polishing using 1.0, 0.3, and 0.05 microns in a sequence. The resulting GCE surface will be mirror-shiny.