32. Surface Preparation for painting Galvainzed Coatings

There are many instances where hot dip galvanized coatings need to be painted. There are well established quality assurance procedures for the painting of hot dip galvanized components in a controlled environment, but it a common requirement to apply paint coatings to hot dip galvanizing under separate contract arrangements or on site.

When a steel item is first hot dip galvanized, its surface is free from oxidation and contamination and is in the best condition for coating. It is also highly susceptible to oxidation, particularly reaction with atmospheric moisture. Most galvanizers 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 galvanized steel.

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

Handling the galvanized items with dirty or oilcontaminated gloves can leave residues on the surface of the galvanizing that will interfere with the adhesion of the paint coating.

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 galvanized 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 galvanized coating.

The following specification is recommended for abrasive blasting of hot dip galvanized surfaces prior to painting.

Compliance with this specification will ensure that not more than 10 microns of zinc will be removed from the galvanized 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 galvanized coating.

Brush or Sweep Blasting Procedures for preparing Hot Dip Galvanizing for Painting

  1. Blast nozzle pressure 40 psi (280 kpa) maximum
  2. Abrasive grade 0-2 - 0.5 mm
  3. Abrasive type - clean limonite or garnet
  4. Distance of nozzle from surface 400 - 500mm
  5. Nozzle type - 10mm 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. With inexperienced operators, a test section should be evaluated by measuring coating thickness before and after blasting with an approved magnetic thickness gauge.

A maximum 5-10 micron reduction in galvanized coating thickness indicates an acceptable technique. Over 10 microns of coating removed indicates an unacceptable technique.

On reactive steel, the coating may already have a matte gray or satin appearance. This indicates the presence of the zinc-iron alloy layers at the surface, which also indicates a thicker than standard galvanized coating. Grey galvanized coatings are more susceptible to mechanical damage than shiny coatings and should be treated accordingly.

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 galvanized coatings of this type.

Weathering of the galvanized surface will generally result in any residual chromate passivation films being removed. However, the galvanized surface can also accumulate corrodents during the weathering period and should be evaluated for cleanliness prior to any coating application.

With weathered galvanizing, free of organic contamination, high pressure water blasting of the surface will often be sufficient to ensure adequate coating adhesion to the galvanizing, given that appropriate primers are used.

Specific abrasive blasting parameters are required when preparing galvanized surfaces for painting. The same techniques used for preparing ‘black; steel will damage the galvanized coating.