I am plastering my new ceiling this weekend. A professional will help me and he is going to use Thistle Multi Finish plaster. The gap in some of the joints is a bit bigger, especially between the ceiling and wall. Shall I also buy Gyproc Easi-Fill 45 or Easi-Fill 20 to fill the joints before plastering? If so which to buy, Easi-Fill 45 or 20?