@Nils
Since my beginning days of heat sink lapping I've settled on my best way to do it, which has taken me from an arduous ordeal to what I call speed lapping, speed lapping is worse for the longevity of the sandpaper but seriously cuts the time involved in the entire lapping job.
I no longer wet sand because it's not necessary, and it is a mess to clean up, I dry sand which will go through your sandpaper faster, but get you where you want to go much faster.
Factors are:
You still want to use automotive grade wet/dry paper, and you don't want to start with a courser grit than the scratches already on the heat sink, and sometimes if you're lapping a HDT (Heatpipe Direct Touch) heat sink, you may need to use a finer starting grit and creep up to the finish so you don't cut any deeper than the existing machined scratches, (More work but less chance of cutting through a heatpipe.).
Always make absolutely positive the heat sink base is flat to the cutting surface, do not allow wobbling during the cutting no matter what you have to do to maintain perfectly flat stability, and some top heavy heat sinks will try to wobble, if vibration begins stop and reposition your grip.
Be sure the surface you're working on is flat and particle free, make sure the sandpaper is also particle free and flat, no folds or creases.
OK here is the speed part, do not tape the paper down to the flat surface, keep the heat sink base about a half inch from the edge of the paper, if you're right handed hold the paper at the top right edge from you with the left hand, pull the heat sink down the side towards you, rotate the paper reposition your left hand, pull towards towards you, continue this process.
After about 3 full rotations of the paper, rotate the heat sink 180 degrees and continue, if you're left handed swap your hand positions and pull the heat sink with the left hand hold the paper with the right, its very simple and very fast.
When the paper starts to build up metal content, carefully vacuum or go outside and shake it off being careful not to wrinkle or crimp the paper and it will cut like brand new.
One final tip is you can actually produce a mirror finish with only 600 grit paper, once you've cut the heat sink base to the point there are no more scratches being removed, stop removing the metal residue from the paper and you'll begin a metal on metal polishing effect, and you can continue to a mirror finish. Good Luck!
Zalman Lap
Xigmatek Lap