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Thursday, July 30, 2015
Coating Galvanized Metal
Newly galvanized metal is a difficult substrate to coat
because it is shiny and smooth with virtually no anchor pattern.
Attempting to coat an un-dulled and excessively smooth galvanized
surface will typically result in coatings failure, especially when these
surfaces are not adequately primed.
The story below highlights the consequences of using a high quality
topcoat, without considering the role of priming and surface
Chipping paint reveals neither primer nor surface
preparation on underlying galvanized surfaces
parts were used in various places around the exterior of a shopping
mall. To achieve the aesthetic vision of the project, these pieces were
topcoated with a high performance latex to ensure long-lasting
After 5 years of
service, some wear and tear (chalking, fading) of the topcoat can be
expected, however the mall owner was not prepared to witness large
pieces of paint peeling off and flaking down to bare metal.
Convinced that the failure was not related to the high performance
latex topcoat, an inspector was called in to determine the cause of
that no primer had been used before the high performance latex topcoat
Typically the primary role of a primer in an exterior metal coating
system is corrosion resistance; however this is not the case for
galvanized surfaces. The zinc galvanizing acts as a protective coating
for the metal, so the main role of a primer in a galvanized metal
coating system is to bond to the surface while promoting adhesion for
lthough the lack of priming may have contributed to the
coating failure, further analysis found that a lack of surface
preparation was likely the primary cause. By examining the surface, the
inspector found that the galvanized metal was still shiny and smooth,
indicating that no preparation was used to roughen the surface.
This failure could have been avoided if the newly-installed galvanized
structures had been washed down with an etch primer (such as those
listed under MPI# 25) to roughen and
dull the surface, was allowed to dry for one day, and then primed with
a waterbased acrylic primer (MPI# 134) before applying
the high performance latex topcoat, allowing the topcoat to fulfill its
expected service life.