Once flat, I was planning on using a diamond hone to polish it flatter.Something to consider is that simply machining the surfaces flat is not flat enough for proper heat transfer. The surfaces should be lapped to a PERFECTLY flat smooth surface otherwise the heat transfer between the old heatsink and the new one will be very poor even when using heatsink compound. We use electrically heated aluminum dies with attached faceplates at work. They periodically get cleaned via glass beading. (sand blasting) Whenever we get a new heater plate in we have to glass bead the surface before putting them into service so that the heat transfer matches the existing ones otherwise we have one die running MUCH hotter than the others due to the mirror-like surface transferring heat so much better to the faceplate. After glass beading the surface still feels perfectly smooth however it is just "rough" enough to affect the heat transfer.