Hi
The case of Apple Watch Sport has an anodized finish: "Each case is machined and polished, then blasted with microscopic zirconia beads to achieve a consistent, satin texture. A special anodizing process creates a hard, clear outer layer that helps protect against scratches and corrosion."
Apple does not recommend polishing or buffing the watch with abrasives:
Cleaning and wearing Apple Watch - Apple Support
Rather than fixing the original scratch, use of any abrasives (including mechanical polishing) will only cause further damage to the anodized finish. There is no way to restore an anodized finish other than by completely removing the coating and re-anodizing the whole case.