When painting hot dip galvanizing, as when painting any other surface, the cleanliness and condition of the surface are of critical importance and a high proportion of paint failures on hot dip galvanized steel can be attributed to inappropriate or inadequate surface preparation.
In preparing hot dip galvanizing for painting, the basic requirements are largely the same as for other surfaces. Namely, anything that prevents the paint wetting out or adhering to the surface needs to be removed. Therefore oils, dirt, dust, salts, corrosion products and other friable material and soluble salts must be removed as a precursor to any subsequent treatment. Refer to AS/NZS 2312.2 Section 7.5.3 and AS 1627.0. The difficulty of removing some contaminants should not be underestimated and for severely corroded hot dip galvanizing, in particular, reinstatement may be impractical, because of the extensive preparation required.
For the removal of oil and grease, water based emulsifiers, alkaline cleaners of pH less than 12 or organic solvents are variously appropriate. Where oils and grease are removed by solvent soaked cloths, these need to be changed frequently as oil contaminated cloths only serve to spread the contamination. Usually this method is only practical over small areas.
Apart from the removal of dirt, dust and grease, which are common to all substrates, it is important to recognise all sophisticated coatings intended for extended durability service require high standards of surface preparation for maximised performance.
One important issue for hot dip galvanized surfaces is the time lapse between hot dip galvanizing and painting. The best advice is to paint hot dip galvanizing as soon as possible, for the sooner it is painted the less likely it is to be contaminated by dust, salts and corrosion products. Conversely, the longer the time lapse and the more severe the conditions of exposure prior to painting, the more difficult and costly the preparation will be. In extreme cases, such as where surfaces have been close packed in humid or damp conditions and suffered wet storage staining, brushing with 1% to 2% ammonia, or in extreme cases one part citric or acetic acid to 25 parts water, may be required.
A second consideration is the smooth, glossy surface that emerges from the hot dip galvanizing bath. This can inhibit paint adhesion. In the past, two methods of dealing with this problem were to etch the surface with an aggressive salt solution or mineral acid or allow the zinc to weather for some time before painting. These techniques have long been discredited.
For painting unweathered hot dip galvanizing with conventional low build paints (see Service Requirements 1 & 2) cleaning and degreasing is normally adequate, although light scuffing with sandpaper will invariably enhance paint adhesion. For higher build paints and under conditions of more arduous wear, brush (whip) abrasive blasting is favoured (see Service Requirements 3 to 6).
This process lightly roughens the surface without removing a significant amount of hot dip galvanizing and provides a key to promote adhesion of the paint film. This procedure should be carried out using a soft abrasive, by impacting the surface at a glancing angle and operating at low air pressure. The following criteria included in AS/NZS 2312.2 Section 18.104.22.168, AS/NZS 4680 Appendix I, and AS1627.4(12) are recommended:
- Blast pressure 275 kPa (40 psi)
- Abrasive Grade 0.2 – 0.5 mm
- Non-metallic, soft abrasive media, e.g., ilmenite or garnet
- Angle of blasting to surface no greater than 45°
- Distance from surface 350 – 400 mm
- Nozzle orifice diameter 10 – 13 mm of venturi type
It is important that this procedure be performed carefully to ensure that no more than 10 μm of zinc is removed. Organic paint coatings should be applied as soon as possible after abrasive blasting.
Newly hot dip galvanized coatings are normally quenched in an aqueous solution by the hot dip galvanizer to impede early onset of white rust. If a hot dip galvanized article is to be painted, it is usually best to exclude this step from the hot dip galvanizing process to minimise contamination. To ensure the best possible surface for painting, advise the hot dip galvanizer of your needs prior to hot dip galvanizing.