Significance and Use
4.1 Wet film thickness measurements of coatings applied on articles can be very helpful in controlling the thickness of the final dry coating, although in some specifications the wet film thickness is specified. Most protective and high performance coatings are applied to meet a requirement or specification for dry film thickness for each coat or for the completed coating system, or for both.
4.2 There is a direct relationship between dry film thickness and wet film thickness. The wet film/dry film ratio is determined by the volume of volatiles in the coating as applied, including permitted thinning. With some flat coatings the dry film thickness is higher than that calculated from the wet film thickness. Consequently, the results from the notch gage are not to be used to verify the nonvolatile content of a coating.
4.3 Measurement of wet film thickness at the time of application is most appropriate as it permits correction and adjustment of the film by the applicator at the time of application. 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.
4.4 The procedures using notched gages do not provide as accurate or sensitive measurements of wet film thickness as do the Interchemical and Pfund gages described in Test Methods D1212. Notch gages may, however, be used on nonuniform surfaces, like concrete block, that are too rough to use the Interchemical and Pfund gages. Also notched gages can be very useful in the shop and field for determining the approximate thickness of wet films over commercial articles where size(s) and shape(s) are not suitable for measurements by other types of gages. Examples of such items are ellipses, thin edges, and corners.
1.1 This practice describes the use of thin rigid metal notched gages, also called step or comb gages, in the measurement of wet film thickness of organic coatings, such as paint, varnish, and lacquer.
1.2 Notched gage measurements are neither accurate nor sensitive, but they are useful in determining approximate wet film thickness of coatings on articles where size(s) and shape(s) prohibit the use of the more precise methods given in Test Methods D1212.
1.3.1 Procedure A—A square or rectangular rigid metal gage with notched sides is used to measure wet film thicknesses ranging from 3 to 2000 μm (0.5 to 80 mils 1). Such a gage is applicable to coatings on flat substrates and to coatings on articles of various sizes and complex shapes where it is possible to get the end tabs of the gage to rest in the same plane on the substrate.