How do I smooth a scratchy nib?

First diagnose: scratchy in one direction usually means misaligned tines (align with fingernail pressure — free, five minutes). Scratchy in all directions on a properly aligned nib is a polish problem.

For polish, write figure-eights on 12000-grit micromesh with light pressure, testing every few strokes — you are removing microns. Never touch coarser grits on a nib you like, and leave gold and specialty grinds to a nibmeister; tipping removed is tipping gone.