when you consider this heat spreader is nickel plated pure copper.........it's amazing there's not more damage considering what!......about 40 to 60 LB/PER/SQ/Inch of pressure has been applied......if you're really worried.......get a white board marker and colour in the bottom of the spreader and the base of the heat sink........wait a minute and wipe it off on a piece of A4 paper on a flat surface..........if 95% comes off .... there's no real problem........this is what a good thermal paste is designed to do ..........if it's substantially less........there's a few high point that need to be knocked off.........post back if you need advise on the procedure............remember to us good antistatic practises.
Aussie Allan