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EPA Self-Audit and Inspection Guide Organic Finishing of Metals SURFACE PREPARATION Proper preparation of the part surface prior to organic finishing is essential to the performance of the final coating. Improper treatment of the part surface can lead to poor adhesion of the coating material, surface defects, and aesthetic problems. The choice of surface preparation technology used will depend on the part substrate, size and shape, as well as its previous manufacturing steps. Surface preparation includes stripping of previous coatings, removal of contaminants such as grease and oils, and drying. More than one process may be necessary to fully prepare a workpiece for a new organic finish. Regardless, almost all parts will go through a surface preparation step prior to coating. Old coating material should be completely removed before refinishing. Applying a new coating over previous coatings can hamper its performance, especially if the old coating is flaking or peeling. The new coating will not adhere to the actual part surface but to the old layer of finish and will flake or peel away from the part. Removal of old coatings also provides a flat, even surface so that the new coating will have a uniform thickness. Other coatings or surface damage, such as weld bums, should also be removed. Coatings removal can be completed in several ways, using chemical or mechanical means to prepare the part surface. Other preparation steps include those where the surface is cleaned and the substrate is activated. Using solvents and water to separate the unwanted materials from the substrate, cleaning removes grease, fingerprints, dirt, and other contaminants that will interfere with the application and curing of the finish. Chemical activation steps promote a chemical reaction on the surface of some metals so that it more readily accepts the coating material. Activation is completed by applying a water-based solution of active ions to the part. These surface preparation techniques are often performed in series with the parts moving from a wash step immediately to a rinse step and then to an activation step. Drying is often required as a last step, and then the parts move directly to the application process. Environmental impacts must also be considered when choosing surface preparation methods. The harsh chemicals and cleaners used to remove contaminants can create a large volume of waste liquids. The residual grease, diM and coatings must also be disposed. Some of these cleaners and residual material may be classified as hazardous material, adding to the burden on the facility. Six surface preparation techniques are described in this chapter: |