This is the Associated Diff Ring ( 12LW). The ring is “cone”when punched out during in the manufacturing process. So, we need to use the micro finishing film to “grind” it flat.
Besides, we need to gently "rough" the surface, so the steel / ceramic balls can grab on the rings instead of slipping.
We use Micro Finishing film because it provide a better Bearing Surface area on the diff rings than conventional abrasive sand paper. But, it is a bit costly.
You diff now run super smooth, and you don't need to over-tighten your nut, the balls also last longer.
