Successfully painting over hot dipped galvanized steel, known as a duplex system, is an orthodox industrial painting procedure, which in Europe runs into millions of tonnes of steel per annum.
The purpose of the process is to:
• create an aesthetic or identifying colour
• extend service life
• add chemical resistance for which galvanizing alone is unsuitable
This system of expanding the fundamental characteristics of galvanizing should not be taken as matching the appearance of painting in terms of high gloss, surface smoothness or paint finishes obtainable over substrates of timber, plaster automotive or white goods sheet metal.
An after-fabrication galvanized surface will retain its cast coating appearance or, in effect, a natural rather than manufactured metallic finish. This distinction is generally the reason for its use to add a specific architectural feature as well as obtain long maintenance free life.
The alternative intention must be clearly understood and not judged by decorative paint finish criteria.
Duplex appearance, in the case of heavy duty coatings, for severe marine or chemical exposure differs little from paint alone and is of little consequence.
After-fabrication galvanizing is most often painted, where in protective terms it is least needed; to introduce the aesthetic effect of colour. Nevertheless, this is a reliable process for which good products are available, however correct paint selection and the same disciplined surface preparation required for any paint job is essential.
A cursory coat of house paint will give a poor result and short life, alien to the otherwise reliability of galvanizing.
Carried out correctly, a good decorative duplex will provide longer service life, the maximum colour service through lack of under film corrosion and a minimal repaint cost, since future steel surface preparation will not be required.
In effect, future maintenance will be simply colour refreshment as a result of paint degradation from solar radiation.
2. Atmospheric Service Life
After-fabrication galvanizing has a very wide atmospheric protection range however, since a quality protective coating provides a directly added service period, the combination is useful in situations of severe coastal exposure although rarely essential, apart from direct surf or close tidal conditions.
In effect such combinations of paints and after-fabrication galvanizing have a synergistic life effect, providing from 1.5 to 2.7 times the sum of their separate life expectancies.
Thus, while galvanizing is not recommended for full immersion in salt water for normal service life, this facility gives it a reliable extension of its recommended adjacent service zone.
3. Specific Chemical Resistance
Galvanizing is used in the ph range between 6 and 12. Outside this range an unacceptable service life will result.
As in the same way as meeting marine conditions appropriately, chemically resistant heavy duty topcoats will provide an efficient option, where combinations of hard wear, chemical resistance and design construct and installation efficiencies may benefit from the combined tolerance of such specialist systems.
This is normally only used in moderate chemical exposure, beyond which all protective coatings carry risks, but this is a sound mechanism to secure galvanizing life close to extremes of ph and other chemical conditions.
After-fabrication galvanizing alone will resist many chemical and aqueous conditions, however specific recommendation should always be sought.
Galvanizing surface preparation
All quality paint work including duplex application is founded on disciplined surface preparation and correct coating selection.
Zinc, like timber, concrete masonry and other substrates, has specific adhesion requirements, which must be met.
Zinc substrates of any kind require careful removal of oil, grease and loose surface zinc salts and oxides, which readily accumulate when handling steel items.
AS1627.1 describes suitable degreasing and cleaning, which should precede any painting of galvanizing.
Very light sweep blasting as per GAA Datasheet GEN1/1 is an alternative, which will be important to provide optimum adhesion for service such as fresh water immersion. The instructions given here and in Appendix 1 of AS/NZS4680, the Standard for After Fabrication Galvanizing, must be closely followed and bear no relationship to sweep blasting steel.
A critical issue with both of these cleaning methods is the time lapse between the surface cleaning and the application of paint. Salt and contaminate build-up commences almost as soon as the preparation process is complete. Delays may result in the failure of any paint coating in such circumstances. The most important specification detail is therefore the initial phase of surface preparation and priming which is similar to that of over coating quality inorganic zinc silicates.
Self etch primers have historically been much used to prime galvanizing, however currently two pack epoxy phosphates are now sometimes used alone and are adhesion promotion products. These are available from protective coating suppliers and have a reputation for reliability. This detail is the most important of all in duplex processing.
Use of an incorrect primer formulation will give poor adhesion since specific adhesion, and not surface penetration, is the mechanism.
2. Top Coats
Given the above, a wide range of top coats are available however alkyd house paints are not suitable. Epoxy, urethane or catalysed acrylics are appropriate.
Paint formulated for high performance steel protection over zinc paints are also usually the appropriate grade for top coating galvanizing. In general paints formulated for other surfaces are unsuitable.
In all paint coating selection paint producer’s recommendations must be closely followed.
Duplex systems of After-Fabrication Galvanizing and paint, is a sound method of extending service life and adding colour to steel. The service life of the product can be exceptional in most circumstances. However a duplex system is only effective where preparation, application and proper choice of coatings have been strictly adhered too.