First of all, if you are dealing with already painted walls, you need to understand what kind of paint has been used previously.
The second thing is to evaluate any detachments (if any) and repair them. In the same way, the signs of humidity must be eliminated; and molds with the right techniques and the right products.
If it is the walls of old rustic houses it will be good to highlight the roughness and undulations.
On the other hand, if it is a matter of walls of modern houses, it will be important to check the perfect flatness of the surfaces and eliminate any drippings or signs of impact.
The whiskers of the heater
The black glazes that start from the radiators can be removed by washing the wall in advance with water and neutral soap or water and bleaching the area in order not to re-emerge the black under the new painting.
In case of obvious pier stains it will be necessary to sand the area with fine sandpaper.