Painting over Galvanizing

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Even in more recent times, basic problems such as the incompatibility of some paints with zinc coatings were not clearly identified, and the common sight of paint peeling off galvanized surfaces; guttering and railing in particular, created an attitude which still prevails, that galvanizing is difficult to paint.

Adhesion in duplex systems depends on the chemical and physical behavior of the underside of the paint film and the surface of the galvanizing. With better surface analysis tools available to the researcher, and the increased awareness of the importance of surface chemistry in the behavior of materials, a clearer understanding of what happens at the interface is assisting in the development of better duplex coatings.

There has been much fundamental research done in this area, and in summary the following forces are considered to be important in influencing adhesion of paint to galvanizing or other metal substrates.

• 1. The bonding forces in the very thin (submicron) boundary zone

  2. The thermodynamic forces relating to the surface energies of the two materials.

  3. Electrostatic forces which are a measure of the energy required to remove a coating from its substrate. The energy involved to do this should exceed that required to break down the molecular bonds in the polymers.

  4. Mechanical adhesion forces.

Thus the mechanism of adhesion is a combination of atomic and molecular bonding, wetting angles, electrostatic and mechanical keying forces. Coating developments focus on each of these areas to improve the adhesion performance of paint over galvanizing.

• 1. Brush blasting of the galvanized surface resulted in very good adhesion of all paints tested.

  2. The epoxy and polyurethane paints did not adhere well to weathered zinc surfaces. Chlorinated rubber, polyvinyl and water borne acrylic paints had acceptable adhesion.

  3. All paints adhered satisfactorily to newly galvanized surfaces.

The Superetch primer is a single pack epoxy etch primer with the following characteristics: