Meaning and Purpose
4.1 Wet film thickness measurement of the coating applied to the item is very helpful in controlling the thickness of the final dry coating, although the wet film thickness is specified in some specifications. Most protective and high-performance coatings are applied to meet dry film thickness requirements or specifications for each coat or complete coating system or both.

4.2 There is a direct relationship between dry film thickness and wet film thickness. The wet/dry film ratio depends on the volume of volatiles in the coating applied, including the allowed thinning. For some flat coatings, the dry film thickness is higher than the value calculated based on the wet film thickness. Therefore, the results of the notch gauge cannot be used to verify the non-volatile content of the coating.

4.3 It is more appropriate to measure the wet film thickness at the time of construction, as it allows the construction personnel to correct and adjust the film during construction. Correcting the film after drying or chemical curing takes a significant amount of additional labor time, may lead to film contamination, and may introduce adhesion and integrity issues in the coating system.

4.4 Procedures using notch gauges do not measure wet film thickness as accurately or sensitively as the Interchemical and Pfund gauges described in Test Method D1212. However, notch gauges can be used on uneven surfaces, such as concrete blocks, that are too rough to use Interchemical and Pfund gauges. Notch gauges are also very useful on the shop floor and in the field to determine the approximate thickness of wet film on commercial items, where the dimensions and shapes are not suitable for measurement with other types of gauges. Examples of such items are ellipses, thin edges, and corners.

4.5 An operator with experience in the use of notch gauges can well monitor the coating application to ensure that the required minimum film thickness is obtained.

4.6 Application losses, such as overspray, transfer losses and paint residue in the application equipment, are a significant non-measurable part of the paint used in the work and are not calculated by measuring the wet film thickness.

ASTM D4414-95-2020 Standard Practice for Measuring Wet Film Thickness by Comb Gauge

Scope of
1.1 This specification describes the use of thin rigid metal notched gauges (also known as step gauges or comb gauges) for measuring wet film thickness of organic coatings such as paints, varnishers and lacquers.

1.2 Notch gauge measurements are neither accurate nor sensitive, but they can be used to determine the approximate wet film thickness of the coating on the item, where the size and shape prohibit the use of the more precise method given in test method D1212.

1.3 This practice is divided into the following two procedures:

1.3.1 Procedure A — Measuring wet film thickness in the range 3 to 2000 μm (0.5 to 80 mille 1) using a square or rectangular rigid metal measuring tool. The measuring tool is suitable for coating on flat substrates and on objects of various sizes and complex shapes, where the end lugs of the measuring tool can be located in the same plane on the substrate.

1.3.2 Procedure B — Measuring wet film thickness in the range 25 to 2500 μm (1 to 100 mils) using a circular thin rigid metal notch gauge. This fixture is suitable for coating on flat substrates and coating on objects of various sizes and complex shapes.

ASTM D4414-95-2020 Standard Practice for Measuring Wet Film Thickness by Comb Gauge

1.4 Values expressed in SI units should be considered standard. The values given in parentheses are for reference only.

1.5 This standard is not intended to address all safety issues, if any, associated with its use. It is the responsibility of users of this standard to establish appropriate safety, health and environmental practices and to determine the applicability of regulatory restrictions prior to use.

1.6 This international Standard has been developed in accordance with the internationally accepted standardization principles established in the decisions of the Technical Barriers to Trade (TBT) Committee of the World Trade Organization on the Formulation of International standards, guidelines and recommended principles.