2000 grit as many mention above is for bling only and is often counter productive to your needs (getting the lowest CPU temp and best OC possible).
I’ll explain: The reason we use thermal compounds like AS5 is to increase the surface area that your CPU and heatsink conducts heat. It does this by filling in the micro lines and holes in the CPU and heatsink to make them flat.
If you’ve sanded your CPU and heat sink to both mirror finishes and for the sake of argument they are both as close to perfectly flat as possible. You then place your thermal compound on as thinly as possible or using the dot/line method. The thermal compound will not squeeze into tiny micro gaps and holes, as there are none. As a result your thermal compound will often act as a insulator and sit between your CPU and heatsink. So rather than help your OC, over polishing can lead to higher temps.