Before starting work, you should cover all the things and places in the room, which can get to the primer. This is very important, because this material has the ability to quickly dry out, and in this form it becomes much more difficult to wipe off. Also, before deciding on the amount of material, you need to accurately find out all the irregularities and disadvantages of the ceiling. Only after that you should start work.
In those places where the irregularities of the ceiling exceed 3 mm, the primer with the greater ability to penetrate should be used. This material will help eliminate irregularities not only visually, but also physically, which contributes to the involuntary leveling of your ceiling.
In order to prepare the coating for subsequent plaster and painting, experts sometimes advise applying several layers of primer, which contributes to the better penetration of this material into the ceiling, which will help you serve the paint much longer than it was originally supposed.
Based on all of what has been said, you need to advise people who make repairs attentive to all elements of the process, because this will help your repair become even better.