Before proceeding to repair the ceiling, it is necessary to completely remove the old coating and carefully inspect the ceiling. All cracks must be expanded with a spatula, a place where the plaster is poorly kept must be cleaned and primed with a primer used to porous surfaces, then let the ceiling dry. After the work done, you can start correcting defects. But if you need to paint more than one layer, then you need to do this only after the previous layer dries.
Deep cracks should be removed with putty with a cement base. For a quick process, you can use a solution of alabaster, but when using this solution, the quality of the repair can suffer significantly. It is also important to glue the joints of concrete plates if there are on the ceiling.
If the ceiling is covered with a coating of a water emulsion, then it can not be completely put up with it. It is enough to limit yourself to putting the spoiled places. The main thing is to remember that the paint in putty and not putty areas can have some differences.
Complete putting the ceiling is necessary if you want to cover the site with water-emulsion paint, which is originally covered with enamel. When the ceiling dries, it must be treated with sandpaper and proceed to applying the coating.