In many cases, after the walls of the walls, as well as some mechanical damage on the plaster, many damage may appear, and therefore it is better to immediately close and repair them.
And first it must be scraped off with a sharp object to the main layer, if the plaster lies firmly, then you can put a patch on the damaged place, on the upper finishing layer. Before this, the surface must be cleaned and a binder is applied to it, in this case, PVA glue is taken. And apply it so that it captures the damaged place, when the glue layer dries, you can apply the finishing layer of plaster. At leisure it will not be superfluous to find out about the site .
If the main layers of plaster are damaged, then the renovated coating can after some time behind the base of the wall. And so that this does not happen, you need to remove the entire plaster. And to determine the places of its detachment, you need to tap the wall with a bone of the finger, and if the plaster exfoliates, then a dull sound will be heard.
To eliminate small defects, you need to make a pile outlet, that is, grouting an old plaster with preliminary smearing a thin layer of solution on it.
But for any repair, the crack should be cut with a knife and cleaned, then wetted with water and filled with a spatula with a solution that formed a hole.