In outdoor service, remote from the coastal fringe and isolated from areas of severe industrial pollution, hot dip galvanizing is inherently durable.
This contrasts with paints and other organic materials, which are degraded by solar radiation. Therefore, in most conditions of atmospheric exposure, little is to be gained from painting hot dip galvanized steel that has a coating thickness more than 42 µm (300 g/m2) unless aesthetic or colour considerations are important (see Figure 2).
In benign internal situations, and particularly conditions of extreme impact or hard wear, unless a change in colour or gloss is considered necessary, hot dip galvanizing is usually best left unpainted.
A great deal of hot dip galvanizing is painted on a casual basis, with conventional latex or suitably primed solvent-based alkyd paint. The choice of primer for alkyd paint is crucial and requires a clear recommendation from the paint manufacturer. In particular, the use of an alkyd primer in direct contact with the hot dip galvanizing risks delamination of the paint due to its saponification.
It is important to note that because these paint systems are quite thin, typically 70 to 120 µm for a three coat system, the zinc profile and localised areas of increased zinc thickness, such as at edges, may be visible in the finish, much in the same way as the grain is visible in painted timber. While this would rarely be a problem, higher build paints can be used as part of the painting specification.
While acknowledging the lesser user requirements of conventional (DIY) decorative paints, strict adherence to the appropriate surface preparation and prime coat specification are a key to reliability in all situations when painting over hot dip galvanizing.
The paint systems detailed under Service Conditions 1 and 2, are essentially decorative paint systems.