Metal Finishing Guide Book


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Page 192 of 903

Disadvantages 1. Preparation requires mixing of two or three components. 2. If coatings are dried in low-temperature and/or high-humidity environments, poor films may form, leading to degraded physical and chemical properties. 3. Surfaces to be coated must be free of traces of oil and grease. 4. Improper surface preparation or mixing can cause edge pull, poor substrate wetting, and cratering. 5. Material handling and application equipment must be of corrosion-resistant materials due to the corrosive nature of the liquid coating. 6. Storage areas should be protected from excessively low temperatures to prevent freezing of the coating. 7. Storage areas should be protected from excessively high temperatures to prevent degradation of the coating. 8. General housekeeping must be rigorous to prevent any bacterial or fungal growth that can interfere with product quality. Plant personnel must be trained in this preventive maintenance. 9. The paint operator may need to go through some form of training to learn how to establish the correct spraying viscosity. AIR AND FORCE-DRIED EPOXIES Water-reducible epoxy coatings are air- or forced-air dried at temperatures less than 90��C (194��F). Advantages 1. VOC levels are at or below 340 g/L (2.8 lb/gal), less water. 2. Primers are available with chromates, or lead and chromate-free. 3. Primers comply with military specifications MIL-P-85582 and MIL-P-53030. 4. The coatings dry to recoat quickly, even in highly humid environments provided that there is good ventilation. 5. Compatible with many types of topcoats, especially polyurethanes. 6. Can be applied with most types of spray equipment, but may not be compatible with airless and air-assisted airless spray guns. 7. Primer is available in only a small range of colors. 8. Topcoats can be made in a range of colors and gloss levels. Disadvantages 1. The coatings are two- or three-component systems comprising a base, a catalyst (curing agent) and, water. 2. The coatings must be applied to properly prepared surfaces. 3. Depending on formulation, it can be difficult to mix the base and catalyst components due to their high viscosity. After adding water, the viscosity drops to manageable levels. 4. The coatings must be mixed using prescribed procedures. 187

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