You can replace thermal pads with apropriate thermal paste which is designed to replace thermal pads. Normal thermal pastes should be of high viscocity in order to cover only the very small gaps between the CPU (or other hot component) and the heat sink so there should be in no way as thermal pad replacement. A thermal paste that is designed to replace thermal pads should be very gummy in order to cover big gaps and of very high thermal conductivity. There is one called K5-PRO which is designed for iMac computers but should be fine for any other computer. You can find videos of it on youtube
http://www.youtube.com/watch?v=e9v5MDivZfc
and it is avaliable on ebay
http://www.ebay.com/itm/261419411897
Just search for
k5-pro thermal paste