Paints & Coatings Resource Center

EPA Self-Audit and Inspection Guide
Organic Finishing of Metals

Table of Contents


Coating application is the second process in organic finishing systems. The coating materials are applied to the workpiece in a variety of ways. Coatings materials can be sprayed over the part, or the part can be dipped into a tank of coating material. Other methods include showering parts with coatings or rolling parts between large barrels to spread on the coating. Transfer of coating materials can be enhanced with the use of electrical potential between the coating and the part.

Several variables dictate the choice of application method. Part geometry, appearance of the coating finish, and production rate all influence the type of application method. A part with recesses and rounded areas that requires a high-gloss finish will be coated by a different system than a flat sheet which needs a protective primer coating. Facility constraints will also determine the choice of application method. The configuration of the application equipment is dependent on space or climate. Systems can be manually or automatically controlled. Other systems may require extra equipment, such as holding tanks or outside air supply to operate properly.

Similar application systems may operate at widely varying parameters. The viscosity of the coating material, the desired thickness of the final coating, and the complexity of the part will determine the best operating parameters for the application method. Thus, part temperatures, dip times, number of coats, or the amount of current used will be different. These operating parameters are carefully monitored by plant engineers to ensure the quality of the coating meets specifications.

One factor that is important to all application methods is the transfer efficiency of coating material onto the part. Transfer efficiency is the percentage of solid coating material used that actually deposit on the surface of the part. The amount of solvent in the coating material is irrelevant. The higher the transfer efficiency, the better, as more coating material adheres to the part and less is wasted. Transfer efficiency ranges from 25% to 40% for conventional spray systems to almost 100% for dip and powder coating methods. Much of the pollution and waste created from organic finishing operations can be minimized or eliminated by improving the transfer efficiency of the application system. If the transfer efficiency cannot be improved, pollution control technology and waste handling measures must be employed.

The environmental issues associated with organic finishing application methods mostly stem from the type of coating material used. More issues arise with the use of solvent-bome coatings than with waterborne or powder coating materials. However, the application processes create pollution and waste that can cause violations but that can be prevented.

This chapter discusses six common application technologies for organic finishing:

  1. Spraying
  2. Electrostatic Spraying
  3. Electrocoating
  4. Roll and Coil Coating
  5. Dip, Flow, and Curtain Coating
  6. Powder Coating

Each section provides a description of the technology and any specific environmental considerations. The final section discusses the regulatory requirements, common causes of violation, sources of pollution, and pollution prevention alternatives for the application methods.

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