yeah you have whats called a thermal pad. Getting those off are usually difficult and beyond that it usually means its an older an less efficient heatsink. I would suggest getting a better heatsink and for better lapping instructions plus available CHEAP kits go here:
they have a kit that goes from 600-800-1000-1200-1500-2000 grit sandpaper and have 10,000grit finishing polish. That'll give you the ultimate mirror finish. Just received my sandpaper kit from them today infact. Good stuff
EDIT: The link he gave the guy jumped from 800 right up to 1500, you'll get a much better finish if you do it incriments that that kit offers.