we (i, actually) stopped lapping chips and heatsinks a few years ago when manufacturers started putting a concave profile to the chip for more surface area and consistent contact. the really hardcore guys would grind chips until the aluminium was nearly gone and the copper or nickel base was visible

i use a big, thick piece of aquarium glass, 1200 for starters and generous shots of wd-40. proper technique on diff rings is easy - 3 fingertips at 10, 2 and 6 o'clock, figure 8's all over the paper, then turn 1 hour and repeat several times. smooth? yes. flat (which is what i wanted)? probably not.