Dealing with Galvanized Surfaces

"Galvanized" describes steel or iron surfaces coated with a thin layer of zinc. While galvanizing alone can provide long-lasting protection, organic coatings may be applied as topcoats either to add color or to provide increased protection in aggressive environments or exposures where repainting operations are not feasible.

When coating galvanized metal, there are many reasons for caution. Different galvanizers use different processes and surface treatments, so what worked with one supplier may not work for another. Also, proper surface preparation is especially critical to assure adhesion on newly galvanized parts: new surfaces are shiny and smooth with virtually no anchor pattern. Other concerns:

• Any "white rust" that forms on galvanized parts exposed to moisture must be removed
• Any residue left from the forming oils used to shape and cut newly galvanized sheets must be removed before painting
• The galvanizer may have applied passivating surface pre-treatments

The Problem with Pre-Treatments

Galvanized metal is sometimes pre-treated by the manufacturer to aid in the galvanizing process or reduce the formation of white rust on the surface during transport or storage. Galvanized metal that has been treated with chromate or similar passivation treatments is difficult to coat satisfactorily, as the treatment will interfere with adhesion.

The specifier should take care to require that the surfaces NOT be passivated if painting is intended, and the galvanizer should be notified in advance that the galvanized steel for the project is to be painted so he knows to not apply a passivating coating.

The specifier should also require the galvanized metal supplier to disclose to the GC or painting contractor any pre-treatment or temporary coatings that have been applied, and specify that these coatings should be removed at no extra cost to the painting contractor.

And the inspector should always test new galvanized surfaces for chromates unless it's absolutely certain that a chromate passivating treatment was not applied.

If present, the passivation treatment must be removed prior to coating. In such cases, refer to the manufacturer for instructions, and in no case should the pre-treatment be considered a primer.

— Excerpted from MPI's on-line training course on painting new surfaces

Revisions to MPI Standards

The following standards were all revised within the last month. The biggest changes were to the gloss requirements for MPI 48, 72, and 10; these have been expanded to cover multiple MPI gloss levels. Click here to purchase and download the revised editions.

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