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ANALYSIS


for more than one coat, due to the depth of applied paint, and since the gauge would need to remain adjusted to a single profile and not used for other projects. Francis further notes that, although this method is still taught in the NACE Coating Inspection programme, its drawbacks were established at least as far back as 1984. Francis concludes: “The method provided by ISO 19840 avoids these weaknesses and are strongly recommended to specifiers and inspectors.”


MEETING MINIMUM THICKNESS Since measuring the entire area of coating is practically impossible, specifications inevitably specify a ‘minimum’ dry film thickness. “To meet this requirement,” Francis continues, “the inspector should in theory check the entire coated surface and mark any areas less than this specified thickness for restoration. The contractor then must build up these areas and continue this process until all readings are above the required minimum thickness. In practice, this procedure is unrealistic for a number of reasons, including: • This approach says nothing about how far out of specification readings may be. For example, a reading 5 microns below the required thickness is of less concern than a reading 50 microns below the required value.


• A large area below thickness is of more concern than a small area. • How close together should readings be taken to ensure the area is completely covered? Should they be every square centimetre be checked, every 0.01 square metres, every 0.1 square metres, every square metre or something else?


• The time taken to measure large areas may exceed the recoat limit, and will undoubtedly cause unnecessary delays to the project.”


Taking this into account, the Standards specify how many readings per unit area are required and what percentage are allowed to be below the specified minimum. However, each has a different approach to these requirements.


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