Duplex Coating: How it Works, How it's Applied

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Preparing Galvanized Steel for Polymeric Coating

The application of polymeric coating on galvanized steel resembles the painting process of steel without any previous coating. Of course, the surface preparation could differ significantly in certain cases, depending on the time elapsed after the galvanization. The ASTM D6386 standard provides the detailed procedures for surface preparation.

Creation of an effective duplex coating involves these steps:

Communicating with customers and others

Evaluating and determining the condition of the galvanized surface

Ensuring thorough cleaning of the surface

Forming the surface profile

Applying the coating and completely curing the coat

Fresh Zinc Coating (within 48 hours of galvanizing) 

Within 48 hours of galvanizing, the zinc coating will be brightly shining or a dull color, and only mild cleaning and profiling will be required for ensuring the adhesion of a polymer coat.

Cleaning in Alkaline Solution

A 10% alkaline solution is used to remove oily and organic contaminants from the surface with minimum damage to zinc coating. If the solution is used in a pressure spray wash, high pressure should not be used. (Maximum 1400 psi.)

Acid Wash

A 4% acid wash is sometimes used for two minutes to remove organic contaminants. Cleaning is done by brush, and rinsing is done twice after the acid wash.

Wash Primer

The wash primer process is based on its acid component. The component reacts with the zinc surface, thus forming a thin film, which adheres to the zinc and the subsequent polymer coating.

Acrylic Treatment

The acidic element of acrylic treatment roughens the zinc coating and creates an acrylic layer to allow subsequent coating adhesion.

Methyl Ethyl Ketene Test 

This test follows ASTM 4752 and is an indicator of a coating's tendency to peel under the influence of a methyl ethyl ketene chemical solvent. MEK is rubbed on the coating 100 times with the help of cotton wool. Any peeling of the coating is carefully noted and recorded. It is an indicator of the coating's solvent resistance.