Hot dip galvanized coatings provide a surface on which it is extremely difficult to apply paint

True or False?

Paint is applied to galvanized surfaces either for added corrosion protection or for decorative purposes. The paint system selected will be determined primarily by the environmental conditions pertaining for each project and the degree of added protection that is necessary.

The synergistic effect obtainable from a correctly applied heavy duty organic coating (duplex system) on galvanized steel is not always appreciated. Extensive studies over many years have shown that the sum of the lives of the paint system and the hot dip galvanized coating is increased by at least 50% when both coatings are used in combination.

The corrosion products formed by corroding steel are between two and three times the volume of steel from which they are produced whereas the corrosion products produced by zinc are only about 20% greater than the original zinc. This fact reduces the propensity for corrosion creep under the surrounding paint film where coating defects are present.

A further benefit is that when maintenance painting becomes necessary, the substrate onto which the paint is to be applied is undoubtedly better than that encountered on an ungalvanized steel surface where red rust is likely to be present at defects in the coating system.

As is the case with all paint coatings, the selection of a compatible paint system is essential when galvanized surfaces are painted. Of equal importance is the need to ensure adequate surface preparation. Inadequate surface preparation is one of the major causes of paint failures regardless of the surface onto which the paint is applied.

There are numerous different generic paints which can be applied directly onto hot dip galvanized surfaces whether for added protection or for aesthetic purposes. Meanwhile, several primers have been developed which enhance the adhesion between paint and hot dip galvanizing. Alkyd enamels must never be applied directly onto a hot dip galvanized surface. This is because the alkyd reacts chemically with zinc to form zinc soaps (saponification) with the result that the paint film delaminates from the zinc surface.

Summary: As is the case for all paint applications, adequate surface preparation is essential. The requirements in the case of hot dip galvanized surfaces are no more onerous than for ungalvanized steel while adhesion properties will be the same if the correct procedures are implemented. An eminent and well-known expert in the field of organic coatings is on record for stating that hot dip galvanizing is the ideal primer on which to apply paint.

The Hot Dip Galvanizers Association is available to provide technical information and recommendations for all projects regardless of how large or how small, where painting of hot dip galvanized steel is required.

back to index