As with all protective treatments of steelwork, it is of great importance that preparation of the galvanized surface is carried out in a thorough and considered manner. In particular, failure to degrease the galvanized steel surface properly is the most common source of failure of duplex coatings.
As with many other substrates, organic coatings cannot usually be applied directly onto galvanized steel. However, simple direct application paint systems, designed specially to adhere to non-ferrous metals such as zinc, are becoming more available in a full range of colours. The reasons for the need for effective surface preparation in most cases are quite straight forward. When the steel is withdrawn from the galvanizing bath it has a clean, bright, shiny surface. With time this changes to a dull grey patina as the surface zinc reacts with oxygen, water and carbon dioxide in the atmosphere to form a complex but tough, stable, protective layer which is tightly adherent to the zinc. The patina takes time to develop and the exact time depends upon the climate around the galvanizing. Typically, the time can vary from six months to two years or more. During this transition of the zinc outer layer into its final state simple oxides and carbonates form, which do not adhere strongly to the surface. As most duplex coatings are applied whilst the galvanizing is in this condition the surface layer must be modified by chemical or mechanical means. Coatings may be applied directly to the initial pure zinc surface or to the weathered surface but the results are not always consistent and ‘taking a chance’ is not recommended.
Where the aesthetic requirements for a duplex system are particularly high, a degree of surface finishing or fettling may be required on galvanized coatings as small surface projections may become more obvious after the application of an organic coating. This is particularly the case for powder coating systems. Care must be taken when smoothing a galvanized coating because the zinc coating may be damaged by heavy or excessive grinding.