Remove the pad that comes on the business end of the FOP-38. Plastic picnic knives don't mar the aluminum. When there's only a little of the thermal pad left, the clear thermal "paste" that comes in a "pen" from Radio Shack removes the last of the pad's crud amazingly well.
Then apply a thin even layer of thermal paste. A coating 1/1000th of an inch thick, on the top of the CPU is ideal. Haven't pulled that one off, yet! Just put it on in the thinnest layer possible, without leaving bare spots.