Drummy Wall Repair, Before: 
A drummy wall, refers to an area of the wall where the plaster has come away from the render and there is normally a large exposed crack because of it, or the plaster is bowed in that area, and the paint has eroded because of it, the plaster must be removed and the render must be prepared using a wet cloth and duster, when that is dry, new plaster is applied with a wall trowel. 
Drummy Wall Repair, After:
After the plaster area repair has dried, normally 24 hours or more, then it is primed, sealed and undercoated with Dulux 1step, then refilled, sanded, dusted, and undercoated and top-coated with the desired colour to match existing walls.