Every chemically homogenous object becomes layered as soon as it assumes any shape or form. The principle of plastic deformation is of utmost importance for the understanding of the behavior and decay of the paint layer. Paint films have plastic properties. These are progressively reduced through cross-linking which replaces the temporary secondary linkages by permanent chemical bonds, which are not susceptible to slippage. Compacting of the paint medium by cross-linking seems to be the prime cause of shrinkage. The crack follows the path of least resistance--through the thinnest and weakest paint layers.