Another way to lap them really well is to glue the sandpaper to piece of glass or solid block of (Flat) hardwood and to move the heatsink against the sandpaper instead of the other way around (Clamp or otherwise fasten the sandpaper block the workarea) It is important that the heatsink surface end up as flat as possible, and holding the heat sink will give you better purchase than trying to hold a piece of sandpaper on a piece of glass or block of wood.
(holding the heat sink; will give; you; better purchase than trying to hold a piece of sandpaper on a piece of glass or block of wood)
holding the heat sink
will give
you better purchase than trying to hold a piece of sandpaper on a piece of glass or block of wood