Lapping/polishing can be done in a way that produces both smoother surface and a flatter surface. The trick is that you must have a surface to work from that is known to be remarkably flat. ....
Polishing compounds are common for final lapping to produce very smooth and flat surfaces. aluminum/metal polish works. It IS like very very fine grit sandpaper.
I played around with aluminium mag polish on an older heat-sink we had laying around from back in the athlon XP days. The surface on that sink was horrendous. It had very large machining grooves in it. I put it on my mill and ran an end-mill across the surface at very high RPM and low feed until all of the previous machine etches were eliminated, (of course, my machining etches remained, but they were remarkably shallow, felt smooth to the touch already) then moved onto polishing with that mag-polish. The result was pretty good, but probably only about as good as it ought to be from the factory nowadays anyways. I suppose if I had started with some very fine grit sand-paper and then gone to the polish the results may have been even better.Edited by mdocod - 3/19/11 at 1:46am