The accurate evaluation of paint film properties is highly dependent on the quality of the preparation of the test sample. Standardized preparation processes minimize deviations caused by human factors and material differences, ensure comparability and reproducibility of test results, and provide a reliable basis for coating development, quality control, and product acceptance. Major domestic and foreign standard systems, such as the International Organization for Standardization (ISO), the American Society for Testing and Materials (ASTM), and the Chinese National Standard (GB), have clearly defined the key parameters for paint film preparation.
Substrate selection and pretreatment
The substrate is the carrier for the adhesion of the paint film, and its selection and treatment are the basis of the preparation work. Commonly used substrates include tinplate, steel plate, aluminum plate, glass plate and plastic plate of specific materials. Selection is subject to the purpose of the test and relevant standards.
At its core, substrate pretreatment is to ensure a clean, flat, oil-free, rust-free, and consistent surface roughness. Typical processes include: solvent cleaning to remove grease, sanding to achieve the specified roughness, re-cleaning to remove abrasive chips, and drying under specified environmental conditions. For special tests, chemical treatments such as phosphating and passivation may also be required.
Coating preparation
The test paint sample must be representative. Samples should be conditioned under standard environmental conditions (e.g., temperature 23±2°C, relative humidity 50±5%) for at least 24 hours prior to testing. For multi-component coatings, they should be mixed strictly according to the ratio specified in the product description and stirred thoroughly. After mixing, it is usually required to stand for a specified period of time to remove air bubbles and should be applied within the specified application period.
Coating method
The coating method is a key factor affecting the thickness and uniformity of the paint film. Commonly used standardization methods include scraper method, spray method, and dip coating method. The scraper method controls the thickness of the wet film through different specifications of gaps, with high reproducibility and wide application. The spraying method requires strict control of the gun model, air pressure, distance, movement speed and overlap rate. The dipping method should specify the lifting speed and the impregnation time.
Regardless of the method, ambient temperature, humidity, and substrate temperature must be precisely controlled. After coating, the sample should be placed horizontally in a dust-free environment with stable airflow, and leveled and flash-dried according to the specified conditions.
Drying and curing process
The drying and curing of the paint film must be carried out in strict accordance with the conditions specified in the product standard or test method standard. The main parameters include: temperature, humidity, time and ventilation conditions. The process is usually divided into air drying and bake drying. Air drying should be carried out under standard temperature and humidity conditions. Baking and drying need to use a blast drying oven, and the sample should reach the specified temperature within the specified time after being put in, and start the time. Under-curing or over-curing can significantly affect paint performance test results.
Paint film thickness measurement
Dry film thickness is the benchmark parameter for most performance tests. Measurements should be taken using a magnetic or eddy current thickness gauge at multiple specified locations on the template, averaging and ensuring that the thickness meets the tolerance range specified by the standard (e.g., ±5% of the specified thickness). Samples with substandard thickness should be discarded.
The prepared sample needs to be cured for a specified time before being put into the test to stabilize the paint film performance. Curing is usually carried out under standard environmental conditions, and the curing time must be clearly stated in the test report.
Summary of key parameters
| Control link | Examples of core parameters |
| environmental conditions | Temperature 23±2°C, relative humidity 50±5% |
| Substrate roughness | The number of sandpaper mesh and Ra value range |
| Coating application period | Time limit from mixing to finish coating |
| Wet film thickness | Controlled by scraper gap or spray parameters |
| Drying conditions | Temperature, time, ventilation method |
| Dry film thickness | Target value and allowable deviation |
| Model maintenance | Maintain the environment and the shortest time |
Conclusion
The standardized preparation of paint film test samples is a systematic process that involves every detail from substrate treatment to final curing. Strictly abiding by relevant standards and specifications and strictly controlling each variable is the fundamental prerequisite for obtaining scientific and reliable test data. This requires operators not only to be proficient in the use of equipment, but also to have a deep understanding of the technical connotation of each step. With the development of materials and technology, the relevant standards are constantly updated, and continuous attention to and application of new standards is necessary to ensure the advancement of testing technology.
References
GB/T 1727-1992 General preparation method of paint film
ASTM D823 Standard Practices for Producing Films of Uniform Thickness of Paint, Varnish, and Related Products on Test Panels
ISO 1514 Paints and varnishes — Standard panels for testing