Answer: yes, it is essential to prevent the wall from "drinking" the paint.
Applying an undercoat is an essential step when you want to paint a wall. Before applying, care must be taken to thoroughly clean the existing wall covering, to fill in any cracks with a filler. For small holes, a smoothing coating is sufficient. Then, you have to sand the surface to level your wall. The undercoat will allow the wall not to "drink" the paint too much during its application. There is no need to lay two underlayers, your wall will not be more uniform. On the other hand, for colored walls, there are gray undercoats which will give depth to the color of your mural. To your brushes!