Small circular aepressions on paint films or craters are caused very often by dusts or enamel drops which comprise surface active materials such as silicone oil. The relations between cratering tendency and such factors as size of dust contaminated with silicone, surface tension, enamel viscosity, and thickness of wet film, were studied. A white enamel with no additives, O. 1% methylsilicone added and 0.3% ethylcellulose added were used for the study. A glass bead of 0.8 mm in diameter and the

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... namel placed in a flat-bottomed beaker of 12 cm in diameter were used as the models of dust and wet film, respectively. The beaker was inclined to obtain the depth of enamel layer of 0.4 to 2. 0 mm. The enamel in beaker was divided into about 6 x 5 sections by lattice-like barriers. The glass bead coated with the methylsilicone was dropped in each section of the enamel layer at regular intervals. The intervals were calculated from the empirical equation, dsldt-z---a-143D, where ds/dt is the increase in enamel solids per min., D the depth of enamel layer, and a, 43 the constants. When the paint layers were sufficiently shallower than the bead diameter, large craters with holes appeared with the enamels without the methylsilicone ; when the paint layers were sufficiently deeper, craters without holes appeared with all the enamels, although the diameters were much smaller with the methylsilicone-added enamel. The diameter reached a maximum at the apparent enamel viscosity of 700-800 poises. With the enamels without the methylsilicone the diameter was increased after the enamel was cured, while with the methylsilicone-added enamel no change was observed. The value ds/dt of the enamel without any additives was considerably affected by the thickness of paint layer as compared with the other enamels.