April 3, 2009

Looking Through the “Recoat Window”

by Tony Ippoliti

As far back as 1965, in the Glossary of Terms Used in Maintenance Painting, the National Association of Corrosion Engineers (today, NACE International)  defined recoat window as the “time interval required between successive coats.”

This “time interval / recoat window” allows the solvents in a primer to evaporate and the protective film to begin to form on, and adhere to, the substrate. More importantly, it suggests WHEN the coating can be successfully topcoated.  It defines a minimum and a maximum “window” during which the applied film can be topcoated. Keep an eye on the recoat window!

Each coating manufacturer has a recommended minimum and maximum recoat window for each of their protective or industrial coatings.  Top coating too soon can result in sags or wrinkles of newly painted surfaces, perhaps because the solvent in the topcoat “re-wets” or “swells” the underlying paint film.

Peeling Paint

Exceeding the recoat window is equally undesirable. With epoxy coatings, used to shop prime water storage tank steel plate, the shop-applied primer gets so hard, solvent- and abrasion-resistant – after a certain period of time has elapsed – that topcoats cannot adhere.  After a water tank fabricator prepares, shapes, and primes steel plate, there remains a certain period of time to apply an intermediate and topcoat in the field. When this time period lapses – when the recoat window is exceeded – peeling and flaking may occur, as seen in this image.

Fortunately, water tank fabricators and painting contractors specializing in water tanks understand this and can overcome the limitations of a coating recoat window. Simply, they do this in accordance with the NACE International & Society for Protective Coatings SP-7/NACE No. 4 Standard: Brush-Off Blast Cleaning.

Want to know more about recoat window or SP-7?

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