Zaratsu is not a Japanese invented technique: it is the phonetic transcription in Japanese of the German surname Sallaz, from the polishing machine manufacturer Gebrüder Sallaz. Japanese master watchmakers of the postwar period adopted those machines and the name became Japanized. Today both terms coexist: Sallaz is used in Europe, Zaratsu in Asia and in specialized English-language press.
The technical principle is precise. The piece is held freehand against the flat front of a rotating disc, not against its rounded edge. This distinction is essential: the flat front introduces no curvature, so the resulting surface remains geometrically flat and free from optical distortion. Scratches, micrometric undulations, any imperfection that scatters light is eliminated. The result is a hard, sharp specular reflection with no halos. The edges remain crisp: that is where the quality of execution is judged, because an edge that rounds off or crosses poorly betrays a hand that has lost control.
Grand Seiko has built its aesthetic on this technique alternated with brushed finishing. Zaratsu polished surfaces capture light with harshness, brushed ones diffuse it: the contrast between the two defines the case volumes far more effectively than any other means. This is clearly visible on 44GS-style cases, where each inclined plane has a deliberate optical function. The same finishing appears on hands and indices, which on certain models are faceted and then polished one by one.
The technique is not exclusive to Grand Seiko, although the brand communicates it more systematically than anyone else. Minase applies it to the facets of the Uruga case. Credor uses it on the thin lugs of certain ultra-thin models. The Seiko Marinemaster 300 receives it because it is assembled in the same Shizukuishi facility.
A detail worth knowing: Grand Seiko produces a proprietary alloy called Ever-Brilliant Steel, with a PREN index (pitting corrosion resistance) approximately 1.7 times higher than standard stainless steel and a whiter tone. After Zaratsu polishing that whiteness becomes even more evident compared to conventional steel. I cannot say with certainty whether the exact composition is public, but the effect on the bench is real and recognizable.
From a practical standpoint: a Zaratsu surface scratches like any other polished surface, and perhaps more visibly precisely because the contrast with the original perfection is greater. Restoring it to its original state requires the same machine and the same hand, not an abrasive cloth.