First, run your fingernail over it to see how uneven it is. If it feels pretty "ridgey" then you might want to lap it.
I went 220>400>600 for my SLK-800...You can take it up to 800grit if you want to make it more of a mirror-like state. Some suggest that going higher makes it better, others suggest that it's not effective over 800grit(including me).
Also, make sure you use BillA's method to lap:
Sand it in 1 direction...do not go back & forth & do not rotate it (move the hs forward, then pick it up and put it back down at its starting point & sand in the very same direction.)
Be very careful to use an absolutely flat surface(like a thick sheet of glass) & put equal pressure on every part of the heatsink or you'll get an uneven lap.
You should take off at least 2-3C from it.
Soy
P.S.
Use a permanent marker on the base to keep track of how much you're sanding...you don't want to sand too much off the base...it also keeps track of any trouble spots that you need to get to. And use AS3 for best results.
