just a mention, the goal is to get the surface as flat and free of imperfections as possible, not to give it a mirror finish.
using a polish like Brasso will fill all the micro-pores in the surfaces of the metal with wax that will impede the function of the thermal interface material.
in other words, you shouldn't use Brasso, Silvo, Any other type of polish or nail varnish remover (contains lingering additives that are good for nails, not good for thermal transfer)
Btw, ~TB~, you DID put the wet'n'dry paper on a sheet of glass to keep it flat, right?