Background

There are many instances where hot dip galvanised coatings need to be painted. There are well established quality assurance procedures for the painting of hot dip galvanised components in a controlled environment, but it is a common requirement to apply paint coatings to hot dip galvanising on site.
Surface Condition

When a steel item is first hot dip galvanised, its surface is free from oxidation and contamination and is in the best condition for coating and is also highly susceptible to oxidation, particularly reaction with atmospheric moisture. Most galvanisers quench the work in a weak sodium dichromate solution to passivate the surface. This chromate passivation film weathers away with time and is replaced by a stable complex carbonate oxide film. This dynamic set of surface conditions needs to be considered when painting galvanised steel.

In addition, surface contamination can occur that will interfere with paint adhesion. Diesel fumes are a common source of surface contamination that are very difficult to detect, as the galvanised coating may still appear clean and bright.

Where close control of surface condition is not possible, the best alternative to ensure a high quality paint application is to brush or sweep blast the galvanised surface immediately prior to painting. This is a poorly understood technique with many paint contractors. Incorrect technique will cause serious damage to the hot dip galvanised coating.
Abrasive Blasting

The following specification is recommended for abrasive blasting of hot dip galvanised surfaces prior to painting. Compliance with this specification will ensure that not more than 10 microns of zinc will be removed from the galvanised coating during the blasting process, and that the coating will not be damaged by fracturing of the alloy layers through excessive impact energy of the blast media on the galvanised coating.
Brush or Sweep Blasting Procedures for Preparing Hot Dip Galvanising for Painting

1. Blast nozzle pressure 50 psi (350 kPa) maximum

2. Abrasive grade 0-2 - 0.5 mm

3. Abrasive type - clean ilmenite or garnet

4. Distance of nozzle from surface 400 - 500mm

5. Nozzle type - 12mm minimum diameter venturi type

6. Blasting angle to surface - 45 degrees

The aim of this blasting procedure is to remove any oxide films and surface contaminants from the surface. It is not to produce a profile similar to that required on bare steel. The brush blasting of the relatively soft zinc will automatically produce a fine profile, giving the clean surface a satin appearance.
Inexperienced Operators

With inexperienced operators, a test section should be evaluated by measuring coating thickness before and after blasting with an approved magnetic thickness gauge. A 5-10 micron reduction in galvanised coating thickness indicates an acceptable technique. Over 10 microns of coating removed indicates an unacceptable technique.
Other Surface Finishes

On reactive steel, the coating may already have a matte grey or satin appearance. This indicates the presence of the zinc-iron alloy layers at the surface, which also indicates a thicker than standard galvanised coating.

The micro-roughness of the alloy layers already provides a good mechanical key for appropriate paint, and only very light brush blasting is required on galvanised coatings of this type.

Grey galvanised coatings are more susceptible to mechanical damage than shiny coatings and should be treated accordingly.