Anodic or Powder Coat Finish?

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Author Message Russell W. Wood, CSI, CCS Senior Member Username: woodr5678 Post Number: 136 Registered: 11-2003 Posted on Tuesday, June 09, 2009 - 09:13 am:    My Architects have designed an 7' high ornamental aluminum picket fence with a gate. Because the gate is subject to touching and therefore abrasion I thought the fence should be anodized. However, the local fence company recommends powder coating. What are your thoughts? Wayne Yancey Senior Member Username: wayne_yancey Post Number: 209 Registered: 01-2008 Posted on Tuesday, June 09, 2009 - 10:36 am:    My thoughts, I anticipate the aluminum fence components (rails and pickets) are joined by welding. Powder coating such sizes after fabrications may be easier than anodizing. Include a clear coat and use bolted connections in the final assembly. Mecedes Benz uses a newly developed (2003) clear lacquer, which contains microscopically small ceramic particles, hardens in the paintshop oven, forming an extensively cross- linked network. The paint is thus more effectively protected against scratches. That said, advantages of anodizing are: 1. In general anodizing is less expensive than painting with the exception of coil painted products. 2. Anodizing is harder than PVDF. Anodizing is better for aluminum in high traffic areas where the coating is subject to physical abuse and abrasive cleaners. 3. Anodizing cannot peel off. The coating is actually part of the metal. 4. Anodizing gives aluminum a deeper, richer metallic appearance than is possible with organic coatings. This is because an anodized coating is translucent, and one can see the base metal underneath the coating. This translucence contributes to color variation problems, but anodizers are doing a much better job of controlling the amount of color variation than in the past. Computerized color matching with quantitative, objective color data is now possible. 5. Anodizing is unaffected by sunlight. All organic coatings will eventually fail due to exposure to ultra-violet light. POWDER COATINGS Liquid paint is composed of pigment, resin, and solvent. Powder paint is simply pigment encapsulated in a powdered resin and is thus simply thought of as "Paint without the solvent. " Powder coatings and liquid coatings made from the same resin and pigment will have practically the same performance characteristics. For a given resin, the decision to use a powder or liquid coating is simply a question of application technique. The real advantage of powder is the reduction in air pollution compared to liquid coatings. When powders are cured in an oven they emit no VOCs. A disadvantage of powder is the large batch sizes that are typically required by powder coatings. The powder manufacturers are aware of this problem and a few of them keep colors in stock that they distribute in small batches. Several companies manufacture exterior grade powders using a Polyester TGIC resin. Polyester TGIC powders are currently available in more stock colors than any other powder. Several companies stock hundreds of colors of Polyester TGIC powders. Some resins are more easily manufactured in liquid coatings and some are more easily manufactured in powder coatings. A particular resin is usually manufactured in either powder or solvent based coatings, but not both. Examples of this are epoxy which is predominantly a powder coating, and PVDF which historically has been manufactured as a liquid coating. Many of the perceived advantages of powders over liquid coatings such as hardness and gloss are actually characteristics of the resin. Powder coatings from most manufacturers are only available in large batches and custom colors can be very expensive. This is because each batch of powder must be ground to order using expensive grinding equipment. Solvent born colors will continue to maintain their niche in the market because of the ease with which small batches can be mixed. The ability to "mix and match" gives painters and their customers unequalled flexibility and ease of use. One note of caution about powders: they are prone to orange peel and the coating may appear to be textured. In order to meet AAMA 2605-98 all organic coatings for architectural aluminum including solvent born and powder coatings require a hexavalent chrome pretreatment. This pretreatment is required for long-term adhesion for exterior use, but it requires that applicators have extensive water pollution control equipment. While this technology is well known and produces excellent adhesion results, the sludge from this process is a hazardous waste and is difficult and costly to dispose.