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October 26 - October 30, 2020

How do you gauge the remaining service life of a weathered alkyd paint system and determine whether it should be removed or overcoated?

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From Wayne Senick of Termarust Technologies on November 7, 2020:
There are two ways to approach this. 1. We can intellectualize and do all kinds of tests. Ie; pull off tests- various types, visual analysis - cracking, peeling, physical or chemical analysis. In most cases this is very costly and subjective and still does not give a definitive answer to the viability of the existing coating, especially the primer. 2. On candidate bridges in most cases we see an 80-20 relationship between good and failing coating (exposed metal). We have been in the overcoating business for 30 years. It has been our experience that any coating which remains after our surface preparation method, [SSPC WJ4 HPWC at 7000 PSI (480 bar), 6 gallons (22.7 liters) per minute with a zero degree rotating tip, a minimum 140 degrees hot water at no more than a four inch standoff distance] is suitable for overcoating. This method has produced long-lasting cost-effective corrosion mitigation solution for structures in the field for many years. After being stressed by this method the remaining coating is tightly adhered and stable enough too be overcoated with an HRCSA rust mitigation overcoating system. As a side bar after surface preparation we have consistently found the remaining substrate adhesion to be more than 500 PSI ASTM D4591 and in some cases as high as 1300 PSI. After the HPWC which is the stress test and used to determine adhesion is suitable for overcoating. The remaining coating after overcoating with a low surface energy overcoating is now being protected by a layer of new coating which acts as a UV blocker and contaminant protection system allowing the remaining coating to continue to do its job at the steel substrate and to mitigate the corrosion taking place on the exposed steel surfaces. In most cases the alkyd systems we encounter are over a existing red lead primer and though in most cases a good percentage of the top coat is removed by the surface preparation the LEAD primer that has not already delaminated (usually 20 percent has delaminated, failed and rusted) the rest is in good shape and when overcoated will continue to protect the surface from corrosion for many years to come. To answer the question, we use our surface preparation method to determine what is good over coat able alkyd coating and what is removable and not over coat able.

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Tagged categories: Asia Pacific; Coating Application; EMEA (Europe, Middle East and Africa); Latin America; North America; Overcoating

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