The following information has been reprinted from the American Galvanizers Association publication "Duplex Systems: Painting Over Hot-Dip Galvanized Steel." The SSPC standard for surface preparation (SSPC-SP 1, -SP 3, -SP 6) should be followed as well.
Surface preparation is a critical factor in achieving good paint adhesion. The two keys to surface preparation are cleaning and profiling the steel. If these two things are done correctly, a compatible paint system should have no problem adhering to the galvanized surface.
When cleaning a galvanized surface prior to painting, the goal is to remove any dirt, grease or oils. At the same time, care must be taken not to remove too much of the galvanized coating. Highly acidic or basic cleaning solutions remove some of the zinc coating, as does high pressure sweep blasting. The more zinc that is removed from the surface, the less corrosion protection provided. Alkaline cleaning, ammonia cleaning and solvent cleaning are the most common ways of removing dirt from a galvanized surface. As some cleaners may react differently with different paint systems, the paint manufacturer should be consulted for specific cleaning instructions.
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 2 to 5 percent sodium compounds with small additions of emulsifying or cheating 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 which are frequently changed in order to avoid respreading of contaminants. After cleaning, thoroughly rinse the surface with hot water and allow to dry completely.
Ammonia Cleaning - A solution of 1 to 2 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 to dry completely.
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. Particle size for a sweep blast of galvanized steel should range between 200 and 500 microns (8 to 20 mils). Aluminum/magnesium silicate and organic media such as corncobs, 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 deg. F with less than 50% relative humidity.
Penetrating Sealers - These products are two-part epoxy sealers forming a coating around 2 mils thick for use on partially weathered galvanizing. Follow manufacturer directions for applications and always use a topcoat over the sealer.
Zinc Phosphate Treatment - This is a conversion coating that can increase the adherence and durability of a paint film. DO NOT USE THESE COATINGS WITH ZINC-RICH PAINTS.
Wash Primers - This treatment uses a metal conditioner to neutralize surface oxides and hydroxides, as well as etch galvanized surfaces in controlled shop conditions. These primers should be applied at a thickness of 0.3 and 0.5 mils (thickness' > 0.5 mils can cause adhesion problems).
Acrylic Passivations - This treatment uses an acidic solution to passivate the galvanized surface and increase paint adhesion. The solution should be applied 0.04 mils thick to a clean surface and allowed to completely dry.