Thixotropic paints should not be mixed because they will lose their viscosity and thin out. When loading a brush, only the first third of the bristles should be dipped in paint to maintain the brush's shape. Cutting in is necessary when painting a window frame to keep paint off adjoining surfaces like the glass. A surface should be keyed before applying a ground coat to promote adhesion. Gold size acts as an adhesive called a mordant for applying gold leaf to a surface.