if your doing it by hand 40 is fine to start out with, but if your using anything pneumatic itll take the filler off ssssooooo quick youll have to refill and start over from scratch.
id start with 80 but thats me. (that i usually do while im sanding on cars)
80-120-240-320-400-600-800-1000-1500-2000 and if you want it perfect you can go to 3000 if you want.
in between the low grits..you should get some guide coating. you spray it on, sand it off. itll tell you if you have any high or low spots..save you alot of trouble later. do that up to about your 320 grit. after that..should be smooth.