The grid of the sandpaper is 800, waterresistant paper!
And make the sandpaper wet with a wet finger( NO drops),
When you grinding KEEP it FLAT.
Check the flatniss, put compound on the CPU (very little rub it with a rasorblade on) and push the HSF to the cpu, and check again the surface of the HSF, There must be compound from the whole core area on the HSF be visible.
IMPORTANT: see instructions on articsilvercompound website, http:/www.articsilver.com/artic_silver_instructions.htm