When painting 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 galvanized steel can be attributed to inappropriate or inadequate surface preparation.
In preparing 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. 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 (Restore Heavy Duty Degreaser), 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 recognize all sophisticated coatings intended for extended durability service require high standards of surface preparation for maximized performance.
A second consideration is the smooth, glossy surface that emerges from the 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 (spirit of salt) or mineral acid or allow the zinc to weather for some time before painting. These techniques have long been discredited. And does more harm than good.
For painting unweathered galvanizing with conventional low build paints cleaning and degreasing is normally adequate (Restore Galvanized Iron Cleaner), although light scuffing with sandpaper will invariably enhance paint adhesion. For higher build paints and under conditions of more arduous wear, abrasive blasting is favoured .
This process lightly roughens the surface without removing a significant amount of 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.
It is important that this procedure be performed carefully to ensure that no more than 10 μM of zinc is removed. Primêr coatings ( Impa MP10 Metal Primer for waterbase or Super Etch Primer for solvent base) should be applied as soon as possible after abrasive blasting.
In conclusion it is recommended that the detailed instructions of each product mentioned above is followed carefully to achieve long-term durability and success.