You want it as smooth as possible before painting. The quality in the paint job honestly lies with the primer.

I'd start around 400 on any generally rough areas then work up to 800-2000.

being 2k you just need a buzzer on a soft pad with 400 dry and that will be fine but make sure you buzz your 240 block scratches out properly or you will see block marks through your car, hence the reason it's better to go 800 to make sure and guide coat every rub/ block

hey guys just a quick question for the auto painters out there.. what grade should the primer be sanded to before the paint is laid? im using just a solid 2 pack ( no clear coat )

Im going to around 800grit wet and dry before painting, but ive been told you only have to go to 400

cheers Wardy