Measuring up
Measuring the thickness of a coating is important in the paint and corrosion protection industry, so surveyors should possess a good knowledge of the appropriate standards.
Inspections during and after the paint application process have become the norm to preserve the life expectancy of bridges, marine vessels, pipelines and water tanks among a wide variety of structures. Specification standards, such as SSPC-PA2, offer guidance and assistance for coating surveyors, describing the procedures required to measure the thickness of a dry film (DFT).
It is one of several important standards developed by the Society for Protective Coatings that should be observed, determining conformance to dry film thickness requirements. SSPC-PA2 provides a uniform way to evaluate coatings on ferrous and non-ferrous substrates using either a non-destructive coating thickness magnetic gauge or an electronic type gauge (as described by ASTM D7091).
SSPC PA2 includes nine appendixes which must be considered when using the document along with coating thickness restrictions levels (1 to 5) which details gauge reading, spot measurement and area measurement.
However, before proceeding to measure the thickness of a coating, it’s considered to be best practice to complete some important operational steps – as incorporated by reference in SSPC-PA2 – notably gauge calibration, verification of gauge accuracy, and gauge adjustment after the acquisition of base metal readings. Correction for base metal readings (surface profile) should be included if required.
Large coated areas may require hundreds of readings to establish conformance with the specification. Here, following the standard can be useful as it details the number of areas to be checked, the size of the areas, the number of measurements per area and what steps to take if spot measurements do not conform to the specification.
When it comes to the size of an area to be checked, the standard breaks it down into three categories: less than 300 sq. ft. of the coated area; 300 – 1,000 sq. ft; and greater than 1,000 sq. ft. The first category will see every 100 square foot area measured for coating thickness; while for those sections 300 – 1,000 sq. ft., three random areas will be selected and measured. Sections greater than 1,000 sq. ft. will involve selecting three random areas from the first 1,000 sq. ft. for testing, plus one additional area for each additional 1,000 sq. ft. The standard provides further details.
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