Using assembly paste with grit isn't a substitute for a properly fitted seatpost and a seatpost clamp which is tightened properly. If the clamp is so tight that you can barely loosen it then something else is wrong.

Again, if the clamp is that tight then it isn't the grease which is making it slip. It would probably slip without grease. Something else is wrong.

Someone needs to check the fit of the post to the seat tube and the operation and fit of the clamp. Grease and paste on the seatpost are secondary issues.