Primers and putties are applied on the paintable surface before the final paint is applied. Before you apply putty or primer you have to sand the wall with sandpaper. Putty is a kind of paste prepared for applying on walls to fill in any minor dents or to level the surface. Since putty is absorbent, a coat of paint applied directly over it would lead to the paint being absorbed unevenly, leading to patches. A primer layer on top of the putty is a must. It takes away the unevenness caused by putty and the final paint comes out perfect. Thus, you get both protection and a good finish.