Painting Hot-Dipped Galvanized Steel Surface Preparation

Alkaline Cleaning
Oil, grease and dirt can be removed by using an alkaline solution in the pH range of 11-12, but not greater than 13. An alkaline solution is nominally 5 to 10 percent sodium hydroxide compounds with small additions of emulsifying or chelating agents. The solution can be applied through dipping, spraying or brushing. If brushing, use a soft bristle brush of nylon, not copper or steel. If dipping or spraying the alkaline solution, the temperature should range between 140 and 185 degrees F. For newly galvanized surfaces, a water-based emulsifier can be used to remove contaminants. After cleaning, thoroughly rinse the surface with hot water and allow to dry completely.

Solvent Cleaning
Mineral spirits, turpentine, high-flash naphtha, and other typical cleaning solvents can be used to clean galvanized surfaces, provided they are applied with lint-free rags or soft bristled nylon brushes that are frequently changed in order to avoid redistribution of the contaminants. After cleaning, thoroughly rinse the surface with hot water and allow surface to dry completely.

Ammonia Cleaning
A solution of one to two percent ammonia applied with a nylon brush can also be used to clean galvanized surfaces, although this method is typically reserved for cleaning parts with ash residue. As a piece of steel is removed from the galvanizing kettle, it may pick up particles of oxidized zinc from the bath surface, otherwise known as ash. Ash residue must be removed prior to painting. After cleaning, thoroughly rinse the surface with hot water and allow drying completely.

Profiling
In order to provide a good adhesion profile for the paint, the galvanized surface must be flat with no protrusions and slightly roughened to provide an anchor profile for the paint system. Filing high spots, sweep blasting, phosphating, and using wash primers or acrylic passivations are the most common methods of increasing the profile of a galvanized surface. Again, care must be taken not to damage the galvanized coating.

High Spots
Any high spots or rough edges should be removed and smoothed out in order to provide a level surface for paint. Use hand or power tools to grind down the high spots. Care should be taken to remove as little zinc as possible.

Sweep Blasting
In order to roughen the typically smooth galvanized surface after cleaning, an abrasive sweep or brush blast may be used. Care should be taken to prevent removing too much of the zinc coating. Particle size for a sweep blast of galvanized steel should range between 200 and 500 microns (8 to 20 mils). Aluminum/magnesium silicate has been used successfully in the sweep blasting of galvanized steel. Organic media such as corn cobs, walnut shells, corundum, limestone, and mineral sands with a Mohs hardness of five or less may also be used. The temperature of the galvanized part when blasting can have a significant affect on the finished surface profile. Sweep blasting while the galvanized part is still warm, 175 to 390 degrees F, provides an excellent profile. Ambient conditions for sweep blasting are recommended to be less than 50 percent relative humidity and a minimum air temperature of 70 degrees F.

Penetrating Sealers
These products are known as a two-part epoxy penetrating sealer that forms a coating approximately two mils thick. They have been used as a surface treatment method on difficult to clean surfaces such as partially weathered galvanized steel. Follow manufacturers directions for application and always use a topcoat over the sealer.

Zinc Phosphate Treatment
As discussed earlier, phosphating is a conversion coating which can increase the adherence and durability of the paint film. The phosphate treatment can be applied by immersion, spray, or soft bristle nylon brush. The phosphate should only be left on the galvanized steel between three and six minutes. The piece should then be washed with clean water and allowed to completely dry. Begin painting when the surface is dry. Do not use phosphate treatments in conjunction with zinc-rich paints.

Wash Primers
This treatment uses a metal conditioner to neutralize surface oxides and hydroxides, as well as to etch the galvanized surface. Wash primers should be applied to the galvanized surface at thickness between 0.3 and 0.5 mils. Thicknesses above 0.5 mils can cause adhesion problems. Because of this, this profiling method is best applied in shop conditions and not in the field. When using wash primers, follow the manufacturers directions for maximum performance.

Acrylic Passivations
This treatment uses an acidic acrylic solution to passivate the galvanized surface, as well as promote paint adhesion. Acrylic passivation products should be applied approximately 0.04 mils thick to a clean galvanized surface. The coating should be completely dry before painting.

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