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What are the differences between ASTM D6386 and the new powder coating specification ASTM D7803?

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These specifications discuss the necessary surface preparation procedures for the application of a top coating such as paint (ASTM D6386Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for painting) or powder coating (ASTM D7803Standard Practice for Preparation of Zinc and Hardware Surfaces for Powder Coating), and are similar in many ways. Each specification has different preparation procedures for galvanized steel that has been aged or weathered for different lengths of time; however, there are some key differences between the two.

The first notable difference is D6386 allows for the application of wash primers (i.e., SSPC Paint Specification No. 27) and acrylic passivation/pretreatment to the galvanized coating prior to paint application. D7803 does not allow for these types of preparation of the galvanized coating prior to application of powder coating.

The second notable difference between the two specifications is D7803 gives guidance for thermal pretreatment or baking of the galvanized steel prior to powder coating, whereas D6386 does not. Thermal pretreatment of galvanized steel is necessary prior to powder coating to prevent outgassing of the galvanized coating during the baking step in the powder coating curing process. Outgassing occurs when moisture trapped in the outer layer of the coating evaporates as the galvanized steel is heated when the powder coating is baked on. The result of outgassing is pinholing or blistering in the powder coating. By preheating the galvanized steel or iron, the moisture evaporates prior to powder coating application, thereby reducing the chance of pinholing or blistering. D7803 gives guidance for oven temperatures for preheating.

The American Galvanizers Association has developed an instructional DVD and guidebook detailing necessary preparation of galvanized coatings prior to powder coating.  A similar DVD and guidebook for preparing galvanized coatings prior to paint application is also available. ASTM D6386 and D7803 can be obtained directly from ASTM International at www.astm.org.


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