Many heatsinks come with a nicely machined interface pad to make a perfect connection with your CPU. However, there are some which look they could use a bit of work to get to the optimum state of shine and flatness.
In order to do so, you'll have to hunt for the right sandpaper, a flat surface and have the know-how. Fortunately, Kustom PC's have made your life easier by packaging all of the required bits in one product; complete with instructions. Step in the Premium Lapping Kit.