Should I clean and re-apply thermal paste to my CPU every time I remove the heatsink? It's a bother to remove and re-apply it, plus I wonder if having fresh paste is worth the risk of handling the CPU (even properly grounded). Does it really matter if I just remount the heatsink with the existing paste?
I use the thin spread method, if the paste is still in good shape, I just re-spread it really thin again, cleanup any small ammounts of goop comming out the sides, and squish it back down again. If it is aged, looking dry, hardening or the cpu did not remove easily , then it is time to refresh it.
Because I will check the "seating" and the metal to metal contact and lean and pressure , by testing it , I get oppertunity to reuse it. When there is a lot of Time that it is removed for, I cover both sides with plastic, then when it is about to go back together, I decide then if I am going to refresh it or not.
There are different themal interfaces that act, and are different, some are not reusable. There is also the thermal setting/shifting, I figure if it "thermally" changed once and I can get it in place again, it can thermally change again :-) . I have not had any problems re-using the normal stuff they sell us.
If your using the "cross" (strips of goop in a X pattern) pea drop (blob in the center squished out) or Line (Line that gets squished out) methods. These methods are used to keep air out, as the pressure is applied the thermal compound squeezes out from central areas, so air is less likely trapped, pushing thermal compound out of areas. All of these methods would prefer the majority of the thermal compound to be removed. Once removed nobody is going to reuse that.