On 04/17/2008 mr. coatsteel posts: Knowledge of the condition of the galvanized surface layer before coating application is critical to producing an effective coating system. There are three general conditions experienced on the galvanized surface layer.

• Freshly galvanized – this condition usually is seen only within the first 48 hours after galvanizing. In the freshly galvanized condition, there are little or no zinc hydroxides or carbonates on the surface.

• Partially weathered – this condition is usually seen from between 48 hours to 2 years of outdoor exposure of the galvanized surface. The surface possesses a thin layer of zinc hydroxides and/or carbonates (patina) on the surface which is not well adhered to the substrate.

• Fully weathered – this condition is typically seen after 2 years of exposure of the galvanized surface, and possesses a thick adherent patina layer on the surface.

In preparing a freshly galvanized surface for painting, no chemical treatments are required. The best approach to surface preparation prior to coating is light sweep blasting while the surface is still between 150° F and 390° F. Sweep blasting creates a surface profile for the coating to create tighter adhesion of the coating to the galvanizing.

Partially weathered galvanizing also requires a sweep blast to remove the lightly adherent zinc patina layer. However, it may be advisable to provide chemical surface treatments to remove any possible contaminants and improve the adhesion of the coating

Fully weathered galvanizing does not require a sweep or brush blast because the tightly adherent zinc patina has an excellent surface profile and is tightly adhered to the substrate. However, a high pressure power wash is required to remove contaminants.

The surface preparation should be such that there is no water soluble contaminants left on the galvanized surface prior to painting. Another factor of importance is outgassing prior to curing of the paint. Blistering and pin holes will be found on painted galvanized steel if the contaminants and degassing problems are not dealt with during surface preparation.
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