Metal Finishing Guide Book


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bic centimeter. It is difficult to relate the mass of the coating to thickness when the substrate is rough or the coating uneven. Laboratories are best equipped to handle this time-consuming and often destructive method. WET-FILM MEASUREMENT Wet-film thickness gauges help determine how much material to apply wet to achieve a specified dry-film thickness provided that the percent of solids by volume is known. They measure all types of wet organic coatings, such as paint, varnish, and lacquer, on flat or curved smooth surfaces. Measuring wet-film thickness during application identifies the need for immediate correction and adjustment by the applicator. Correction of the film after it has dried or chemically cured requires costly extra labor time, may lead to contamination of the film, and may introduce problems of adhesion and integrity of the coating system. The equations for determining the correct wet-film thickness (WFT), both with and without thinner, are as follows. Without thinner: Fig. 5. Typical coating thickness calibration standards. WFT = desired dry film thickness % solids volume With thinner: WFT = desired dry-film thickness of solids by volume/[100%] + % of thinner added] There are four types of gauges: notch, lens, eccentric roller, and needle micrometer. Each has its own unique operating procedure. Notch gauges, also called step or comb gauges, are most common. They are inexpensive field gauges, which can either be thoroughly cleaned after each reading and reused or disposed of. The notch-type, wet-film thickness gauge is a flat aluminum, plastic, or stainless steel plate with calibrated notches on the edge of each face. The gauge is placed squarely and firmly onto the surface to be measured immediately after the coating application and then removed. The wet-film thickness lies between the highest coated notch and the next uncoated notch. Notched gauge measurements are neither accurate nor sensitive, but they are useful in determining approximate wet-film thickness of coatings on articles where size and shape prohibit the use of more precise methods, such as the lens and eccentric roller gauges (refer to ASTM D 1212). The gauge should be used on smooth surfaces, free from irregularities, and 606

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