^ I checked mine with a razor blade and some light.. I held the razor blade at a 90* angle to the surface of the heatsink at varying positions and look for light between the cut straight edge of the razor blade and the heatsink. If you can see light, it's not TRUE flat..
Stepping the grit of the sand paper will start to polish the surface. 200, 400, 600, 800, 1000gt, 1500gt then 2000 gt.. The 1000+ grit will start to polish the surface. As long as the surface is free of the heavy scratches, you will be fine. Just go up to 1000 or 1500 grit to finish polishing the surface.
I've been polishing things for a while now, so being a gearhead by DNA, this is nothing new to me.
I polished all of my AC Components under the hood of my Camaro.
This WAS dull aluminum. But I used polishing compound after the 1500 grit, though.. I was re-installing the system and a new reciever/drier and decided to shine them up.