Paint losses are a common problem in easel paintings, and their treatment demands both aesthetic and structural considerations. Several filling formulations used throughout history for the completion of missing areas of the ground and paint layers in easel paintings are discussed. Since literature on fillers normally refers to their handling properties, this research is focused on the study of their mechanical and dimensional behavior. This paper will also show how environmental fluctuations affect the properties of traditional fillers (made of skin glues), formulations and those commercially manufactured, and will discuss the strength of these materials and how to determine the allowable relative humidity range for their proper structural stability