Successfully painting or powder coating hot-dip galvanized steel, also known as a duplex system, does not have to be difficult or confusing. Just like painting or powder coating over bare steel, proper surface preparation is crucial to ensuring effective adhesion. Additionally, slightly different methods may be recommended according to the condition of the galvanized surface (newly galvanized, partially weathered galvanizing, fully weathered galvanizing). Refer to ASTM D 6386 for more information.
The two keys to proper surface preparation are:
Other considerations include:
If hot-dip galvanized steel is going to be painted or powder coated, the galvanizer should be notified. Sometimes galvanized steel is passivation-quenched immediately after galvanizing. Quenching solutions interfere with paint and powder coat adhesion and must be removed prior to painting. Notifying the galvanizer you intend to coat the steel after galvanizing alerts him to eliminate the quenching step.
Whichever method of preparation (listed below) is used, care must be taken not to remove too much of the zinc coating. Highly acidic or basic cleaning solutions remove some of the zinc coating; as does high-pressure sweep-blasting. Naturally, the more zinc removed, the less corrosion protection provided.
There are many paint formulations compatible with hot-dip galvanized coatings, but because paint formulations are constantly changing, paint manufacturers should be consulted regarding which of their products are best suited for duplex coating steel.
Mineral spirits, turpentine, high-flash naphtha and other typical cleaning solvents can be used, provided they are applied with lint-free rags or soft-bristled, nylon brushes frequently changed to avoid re-spreading contaminants. After cleaning, surfaces should be thoroughly rinsed with hot water and allowed to dry.
Because some solvents may react differently with different paint systems, consult the paint manufacturer for specific cleaning instructions.
Alkaline Solution 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. The solution can be applied via dipping, spraying or brushing. If brushing, a soft-bristled, nylon brush is recommended. If dipping or spraying, the solution temperature should be between 140-185 F (60-85 C).
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 complete drying.
A solution of 1-2% ammonia applied with a nylon brush can also be used, although this method typically is reserved for cleaning parts with ash residue. After cleaning, thoroughly rinse the surface with hot water and allow complete drying.
In order to provide a good adhesion profile for the paint, the galvanized surface must be free of protrusions and slightly roughened to provide an anchor profile. Hand- or power-tools can be used to remove any high spots. Care should be taken to remove as little zinc as possible.
In order to roughen the typically smooth galvanized surface, an abrasive sweep- or brush-blast may be used. Blast material particle size for profiling galvanized steel should range between eight-20 mils (200-500 microns). Aluminum/magnesium silicate has been used successfully to sweep-blast-profile galvanized steel. Organic media such as dried corncobs, dried walnut shells, corundum, limestone and mineral sands with a Moh’s hardness of five or less may also be used.
The temperature of the galvanized part when blasting can significantly affect the finished surface profile. Sweep-blasting while the galvanized part is still warm (175-390 F [79-199 C]) provides an excellent profile. Recommended ambient conditions for sweep-blasting are less than 50% relative humidity and a minimum temperature of 70 F (21 C).
Penetrating sealers are typically a two-part epoxy that forms a coating approximately two mils (50 microns) thick. They have been used as a surface treatment method on more difficult-to-clean surfaces such as partially weathered galvanized steel. Follow manufacturer directions for application; always use a topcoat over the sealer.
Phosphate solution is used as a conversion coating that can increase the adherence and durability of the paint or powder coating. The phosphate treatment can be applied by immersion, spray or a soft-bristled nylon brush. The phosphate should only be left on the
galvanized steel between three to six minutes. The piece should then be rinsed with clean water and allowed to completely dry.
Wash primers use a metal conditioner to neutralize surface oxides and hydroxides, as well as etch the galvanized surface. Wash primers should be applied to the galvanized surface at thickness between 0.3-0.5 mils (eight-13 microns). Thicknesses above 0.5 mils (13 microns) can cause poor adhesion. Because of this, wash primers are best applied in shop conditions, not in the field. Follow manufacturer directions for maximum performance.
Acrylic passivation is the application of an acidic acrylic solution to passivate the galvanized surface and promote paint or powder-coat adhesion. Acrylic passivation products should be applied approximately 0.04 mils (one micron) thick to a clean galvanized surface. The coating should be completely dry before painting or powder coating.
Determing the Presence of Passivation Agents
Although some passivation-quenched galvanized surfaces have a duller, matte gray finish, it can be difficult to visually detect whether passivation products have been used. If there is uncertainty regarding whether the steel was quenched, the galvanized surface needs to be tested for the presence of passivation agents. This testing is fairly clear-cut and is described in ASTM B 201.