WEDNESDAY, JANUARY 8, 2014
In Western Canada—and, we surmise, other parts of North America—new steel doors are typically galvanized as part of the manufacturing process.
In theory, galvanized doors need not be painted after installation; the galvanizing can provide long-lasting protection from normal weathering and general wear and tear.
But organic coatings are often applied as topcoats to add color for aesthetic reasons (if the shiny silver galvanized surface is not the finish the architect desires), camouflage, or safety; to provide increased protection in aggressive environments; or to extend life in milder environments.
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| Photos: MPI |
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Galvanizing can be painted, but a variety of factors can compromise coating adhesion and performance. |
Specifying topcoats for galvanizing is sometimes referred to as a "duplex" system.
What to Watch For
When painting newly galvanized surfaces, there are several factors to watch out for.
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This is not the look that most architects are seeking with metal doors. |
Priming Galvanized Surfaces
A typical spec that works well for most newly galvanized doors is a wash with MPI# 25 Cleaner, Etching, for Galvanized Metal, which cleans the surface of residual oils and has a phosphoric acid component to etch the surface to enhance adhesion of the primer (MPI# 134 Primer, Galvanized, Water Based).
A wide range of intermediates/topcoats may then be applied, from MPI’s waterborne light-industrial finishes, to high-performance architectural latex, to waterborne alkyds, or even epoxies for aggressive environments.
Unlike most primers for metal, the design purpose of MPI #134 is not to provide corrosion protection. That’s the role of the zinc galvanizing.
Zinc protects the steel by performing a sacrificial function. Less “noble” than steel or iron, the zinc layer will decompose or corrode in preference to the metal substrate, serving to protect the substrate until the zinc is depleted.
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When painting new galvanizing, watch out for surfaces that are too shiny and smooth for the paint to adhere. |
The MPI #134 serves instead as a bonding primer. Its chemistry is designed to:
The Case of the Corroded Doors
This inspector has recently seen two commercial store/warehouse facilities where the faces of the new galvanized doors were so badly corroded that we wondered why the general contractor bothered to hang them.
If you were shopping for a few new doors, you wouldn’t pick these out of the stack! The rusted surfaces had such a pronounced corrosion texture that no conventional paint system could hide it. When the architect inspected the finished work, he’d find his new doors looking as if they’d been repainted repeatedly for decades.
How can new doors look this bad?
The pattern and pervasive nature of the corrosion led the inspector to determine that the doors had been stacked on top of each other in a damp environment for a long time before delivery, most likely while waiting to be shipped from the galvanizer’s or manufacturer’s facility to the jobsite.
Water had apparently found its way between the door skins, ponded on the horizontal surfaces with no means of escape, and caused the rapid corrosion of the zinc metal to the extent that bare steel was exposed and now was rusting.
Suspect Storage
It’s not uncommon for us to find new construction components such as cementitious composition board (aka “Hardy board”) or wood doors stored improperly on a jobsite for weeks or months before usage. But in this case, the GC never had a chance: The improper storage occurred before the doors ever got to the site.
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Improperly stored, newly galvanized doors can develop severe corrosion. |
When the inspector expressed his concern to the General Contractor, the GC made it clear that he was not about to change or replace the doors.
Indeed, in our experience, once doors are hung on a project, it’s very hard to get them replaced or fixed. The inspector brought the painting contractor into the discussion with the GC, but neither was keen to do the surface preparation required to remove the rust and level the surface of the doors.
We reviewed the responsibilities and found that, yes, the painters were responsible to clean and etch the doors. But they were not responsible for the power sanding and grinding that would be required to achieve a rust-free, smooth surface for painting.
So, the general contractor agreed to prepare the doors with his own laborers.
Prime Problems
The work involved a lot of power sanding and grinding, which left the inspector with an interesting problem: The smooth door surfaces were now a mixture of exposed steel and intact galvanizing. The original paint spec called for MPI #134 to be applied–but remember, #134 is not meant to provide corrosion protection.
So while a #134 primer would have been appropriate for the door areas where the galvanizing remained intact, the exposed steel areas—bereft of their zinc coating—would be subject to premature corrosion in the wet, salt-laden Pacific Northwest air. These areas required a primer like MPI #107 Primer, Rust-Inhibitive, Water Based.
As the inspector knows all too well, a surface can only have one “primer”; you can’t apply the #134 over the surface and then apply an anti-corrosive primer. And it’s not plausible to apply the #134 over the galvanized areas and the #107 over the bare areas. What to do?
Two-Fers
Fortunately, some paint manufacturers have double-duty products that serve both purposes: These primers adhere to galvanizing and act as bonding primers for the midcoat, plus they’re formulated with anti-corrosive properties.
The inspector consulted the Approved Products List for both MPI #134 and MPI #107 and, sure enough, found several products approved under both standards. One was selected, applied, and finished with two coats of MPI# 163 Light Industrial Coating, Exterior, Water Based, Semi-Gloss (MPI Gloss Level 5).
The work was found to be acceptable—albeit with unexpected delays to the project and unanticipated expense to the GC.
The Moral of the Story
While galvanizing can offer excellent corrosion protection in service, galvanized steel surfaces will corrode quickly if they are improperly stored—for example, sandwiched together in a horizontal stack for long periods of time in an environment that invites and traps moisture.
Doors should never be stored outside, and indoor storage should be in a dry environment—not a leaky container or shed in a highly humid area, where moisture can condense and pond with little means of escape.
As this project demonstrated, improper storage can cost money and time.
Editor's Note: This article was written by PQA Inspector Dave Lick and is reprinted with permission from the MPI (Master Painters Institute) newsletter. MPI content describes best practices for commercial, institutional, and light industrial painting.
Tagged categories: Coating failure; Galvanized steel; Galvanized/thermal spray coatings; Good Technical Practice; Master Painters Institute (MPI); Paint application
