Zinc Dust Zinc Oxide Coatings

Zinc dust/zinc oxide paints, also known as metallic zinc paints, contain a pigmentation of approximately 80% zinc dust, 20% ZnO, and approximately 80% pigment by weight. These paints offer excellent rust-inhibitive properties, adhesion, film distensibility, and abrasion resistance. Because they adhere tightly to zinc and other metals, they are ideal for prime and finish coat applications and may be used as a primary first coat even over partially rusted surfaces.

Zinc dust/zinc oxide paints are used for the protection of many types of steel structures under a variety of service conditions. They are particularly well suited to use on galvanized steel; are highly satisfactory for priming steel for atmospheric and underwater exposure; and can be used on many outdoor structures, such as bridges, water tanks, and dams, where rusting must be prevented.

In accordance with Federal Specification TT-P-641 which covers primer paints and zinc dust/zinc oxides for galvanized surfaces, there are three types of zinc dust/zinc oxide paints:

• Type I: zinc dust/zinc oxide linseed oil for outdoor exposure, recommended as a primer or finish coat for broad, general use, especially when there is widespread rusting of the steel surfaces; should be air dried only

• Type II: zinc dust/zinc oxide alkyd resin paint, a heat-resistant paint sometimes sold as a stack paint, may also be used for outdoor exposures where rust is not severe; quick drying; can be air dried or baked at temperatures to 150 °C (300 °F)

• Type III: zinc dust/zinc oxide phenolic resin paint; used for water immersion and other severe moisture conditions; may be air dried or baked at temperatures to 150 °C (300 °F)

These paints, when properly formulated and prepared, can be applied by brushing, dipping, or spraying. Although the presence of ZnO prevents rapid or hard settling, adequate agitation of the paint in the dip tank is necessary to ensure the coating homogeneity necessary for maximum metal protection. Pressure equipment should be used when spraying, and the distance between the paint reservoir and the spray gun should be as short as possible to ensure the proper rate of feed to the nozzle. Again, some agitation of the paint in the reservoir is recommended.

Zinc dust/zinc oxide paints possess high covering power and can hide backgrounds of almost any color when spread at the rate of approximately 20 m2/L (800 ft2/gal). However, because the protection afforded by a paint coating is directly related to its thickness, the necessary protection cannot be guaranteed unless the dry film is thick enough for the specific environmental conditions. Therefore, care must be taken to avoid spreading the paints too thin. The natural blue-gray color of zinc dust/zinc oxide paints provides an aesthetic appearance, but if another color is desired, red, buff (orange-yellow), and green can be obtained by varying the pigment.

To prepare surfaces for zinc dust/zinc oxide paints, rust (or scale) and any accumulation of leaves, dirt or other foreign materials should be removed. This may be accomplished on large structures by sandblasting and on small structures or areas with a deck or wire brush.

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