Metal Finishing Guide Book


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coating materials and application methods INTRODUCTION TO PAINT APPLICATION SYSTEMS BY RON JOSEPH (1944-2011) RON JOSEPH & ASSOCIATES INC., SAN JOSE, CALIF. AND MICHAEL MURPHY CONSULTANT, KILLIAN ENTERPRISES, VALLEY COTTAGE, N.Y. A typical line for organic finishing of metals includes the following steps: precleaning, pretreatment, dry-off, primer application, flash-off and/or cure, and final cure. In some cases a primer or intermediate coating may not be used, and multiple applications of topcoat are not unusual. The combination of a pigmented basecoat, often containing metallic flakes, and a final clear, lustrous topcoat is common today. PRECLEANING Procedures for preparation of metal parts for painting often include a precleaning step. Removal of gross soils such as oil, grease, scale, weld spatter, lubricants, and other materials can be accomplished by mechanical means, with solvents, acids, or alkaline cleaning agents. Blasting, polishing, pickling, and solvent degreasing are described in the Metal Finishing Guidebook and Directory Issue. While the purpose of precleaning is to remove the contaminants from the surface, this process in and of itself does not provide any protection to the metals. PRETREATMENT The term pretreatment refers to a combination of chemical cleaning and conversion coating. The purpose of a conversion coating is twofold. It improves corrosion resistance and provides a surface more accepting of the organic coating, subsequently enhancing adhesion. Further details will be found in the pretreatment section of this Guidebook. DRY-OFF After pretreatment the wet parts are immediately transferred into a high-temperature dry-off oven. For lightweight metal parts the oven temperature can be as low as 250°F; however, if the parts are heavy and bulky, the oven temperature may be as high as 204°C (400°F). The sole purpose of this oven is to evaporate off the water as quickly as possible to prevent flash rusting. In coating operations in which the parts are immersed into a dip tank containing a waterborne coating, the dry-off oven may sometimes be eliminated. COATING APPLICATION METHODS There is a wide choice of application methods available for both primers and topcoats. In addition, consideration must be given as to whether manual or automatic techniques should be used. Local regulations may also limit the choice of equipment. Some states, notably California, regulate a minimum transfer efficiency of 156

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