To fix a crack in the plaster, we need fiberglass putty, and normal putty.
First with a spatula we make the crack larger so that the putty fits well, now we take the fiberglass putty and fill the gap with a spatula, leaving the surface as smooth as possible, it will probably be necessary to give two coats of fiber putty of glass.
When the fiberglass putty has dried, we take normal putty and putty on top to make it completely smooth.
Once the normal putty dries, we sand and we can paint again as we want.
In plaster ceilings many slits are usually made, especially between the junction of one plate with the other, but their fix is easy.
And if you have a very large ceiling full of cracks and you want to fix it, it is best to veil the entire ceiling, it is the definitive solution, click here to see how it is veiled.