Accurate measurement of coating dry film thickness is essential for quality control and evaluation of coating properties. This paper will introduce four different methods, including magnetic thickness gauge, micrometer, metallographic microscopy and borehole failure microscopy, for the determination of coating thickness of color coated plates.

Magnetic thickness gauge method
Scope of application: This method is suitable for the determination of coating thickness of color coated plate such as cold rolled plate and galvanized plate, but is not suitable for the coating thickness less than 3μm.

The micrometer method
Scope of application and description: The method is suitable for the determination of the coating thickness of the color coated plate with various materials as the substrate, but it is not suitable for the coating that is easy to deform under the load of the micrometer measuring device, nor is it suitable for the case of the coating thickness is too thin, according to the ASTM D 1005 standard, the film thickness shall not be less than 12.7μm.

Instruments and Materials:

Micrometer: including lever micrometer or digital display handheld micrometer, the instrument value error should be within the range of ±0.001mm.

Solvent or paint stripper (for removing the coating).

A marker.

Method for determination of dry film thickness of color coated plates

Determination method:

Perform zero calibration and operate according to the instrument instructions.

Select 3 different parts for marking in the area not less than 10mm from the edge of the template.

Use a micrometer to measure the coating thickness at the marked site and record the value.

Use an appropriate solvent or paint stripper to remove the coating from the marked area, ensuring that the specimen will not be deformed, bruised or scratched during the film removal process.

Use a micrometer again to measure the thickness of the substrate after removing the coating and record the value.

The result indicates that the coating thickness difference of each measuring part is calculated, and this value is the coating thickness of the part. Then, the arithmetic average of the film thickness at three different measurement sites is taken, expressed in microns (μm).

Method for determination of dry film thickness of color coated plates

Metallographic microscopy
Scope of application and description: This method is suitable for the determination of coating thickness of color coated plate with various materials as the substrate, and can also be used for arbitration test. In this method, the coating thickness is measured by using the different optical reflectance of the coating and the metal substrate on the cross section.

Instruments and Materials:

Metallographic microscope, measurement accuracy should be better than 2μm.

Appropriate grade of metallographic sandpaper.

The material used to fix the test plate shall have no damaging effect on the coating, and the color shall be clearly distinguished from the coating.

Determination method:

Use appropriate material to secure the test plate and ensure that the test plate is perpendicular to the viewing surface.

Polish the test plate so that the surface is smooth enough to view the coating section under a microscope, taking care to keep the test plate at right angles to the sandpaper surface.

The thickness of the coating at 5 different parts of the specimen section was measured using the scale on the metallographic microscope and the values were recorded.

The result shows that the arithmetic average of the coating thickness at five different positions is calculated, and this value is the coating thickness of the test plate, expressed in microns (μm).

Method for determination of dry film thickness of color coated plates

Borehole failure microscopic observation method
Application scope and description: This method is suitable for the determination of the coating thickness of color coated plate with various materials as the substrate, and can also be used for the determination of the thickness of different coatings (primer, top paint) respectively.

Instruments and Materials:

Microthickness gauges, including automatic drilling devices and microscopic video image systems, should measure with accuracy better than 10% of the coating thickness.

A marker.

Method for determination of dry film thickness of color coated plates

Determination method:

Place the sample on the drilling table and adjust the drill depth control wheel of the automatic drilling device so that the drill bit penetrates the substrate just enough to drill a round shallow corner shrinkage hole in the sample.

The sample is placed on the measuring table, and the micro-video image system is adjusted so that all surfaces of the coating at the shrinkage hole can be clearly imprinted on the video.

The thickness of each coating is directly read by the scale of the micro-video image system.

The result shows that the arithmetic average of at least 3 different measurement positions in the measured shrinkage hole is the coating thickness of the sample, expressed in microns (μm).

These four methods provide different ways to measure the thickness of the coating dry film, and the appropriate method can be selected according to the specific situation to ensure the control of coating quality and performance. In practical applications, choosing the right measurement method will help ensure that the product meets specifications and improve the performance and durability of the coating.