White Cement Based Wall Putty HomeSure Wall Putty is a white cement based polymer modified wall putty specially developed as a finish coat to cover unevenness and pinholes in plastered surfaces and to make the walls/ceilings ready for painting. As it is white, it can be treated as a base coat for painting. What is a skim coat? : A skim coat is a thin layer of cementitious plaster which is “skimmed” e.g. applied very thinly onto the interior and exterior of plastered and concrete surfaces.