It can be assumed that this is an older plaster, made in the classic way. Remove any loose, bulging or otherwise damaged plaster. First put a spray, a little cement and a little sand plus water. Very rare. After drying / it's a while / rough. Clay sand, lime. For 5 parts sand 2-2.5 parts lime. There may be a little cement. Not much, less than 1/2 part. Throw, smooth. After hardening, grind / may appear in the new plaster small cracks. It doesn't matter. They stretch stucco / sifted fine sand with lime. Stretch, after partial drying , filter with a felt planer. Small cracks in the plaster are best done before applying the paint, repair with silicone, or even better with a putty. It is also in small packages. You can also buy a finished plaster, both rough and stucco, but if you only have a bit of it, it is better to ask a builder or mix according to the procedure. Before the painting itself, I recommend soaking the entire walls properly and scraping everything that allows it with a flexible spatula.