The Pettit compatibility chart says that certain paint can go over certain other paint, but different levels of sanding are required. Other combinations are labeled "remove paint." Here is the chart:
I can guess that light sanding is probably that sufficient to remove any older loose paint and smooth any ridges. But how far does "heavy sanding" go? I would think not so far as to remove the old paint because then the chart would say "remove paint."
I am looking to overcoat Micron with either Vivid (requires heavy sanding) or Ultima SR-60 (requires light sanding). I suppose the easy choice is to take what requires light sanding, but I have been wondering.