Overview
A wet film preparer is a laboratory tool used to prepare wet films with a specified thickness on a flat substrate. Its core function is to evenly coat the paint into a wet film layer of predetermined thickness through one or more scraping teeth with fixed gaps. This operation is a key prerequisite step for subsequent dry film performance tests, such as adhesion, hardness and weather resistance evaluation.
How it works:
Wet film preparers are typically made of metal or plastic and have a body of a scraped block with precision-machined grooves. As the preparer moves over the substrate at a constant speed and pressure, the excess paint is scraped off and the paint left on the substrate forms a wet film corresponding to the depth of the grooves. Wet film thickness (WFT) is mainly determined by the nominal depth of the preparer groove and can be theoretically correlated with dry film thickness (DFT) by formula: DFT = (WFT × volumetric solids) / 100. The volume solids are the intrinsic parameters of the coating formulation.
Operation process
The operation process begins with the cleaning and fixation of the substrate. Pour a sufficient amount of paint onto one end of the substrate, making sure to cover the front of the area to be coated. Hold the prep so that it is perpendicular to the surface of the substrate and scrape it all the way at a uniform and stable speed. Check whether the wet film is continuous, uniform and free of scratches immediately after scraping. Key control points include: ambient temperature and humidity must comply with relevant test standards; The scraping speed should be kept stable to avoid pause; The preparer should be thoroughly cleaned immediately after use to prevent residual paint from curing and affecting accuracy.
Influencing factors
The actual thickness of the wet film is affected by multiple factors. The main factors include: the processing accuracy and wear state of the preptor, the operator's technique and speed, the rheological characteristics of the coating, and the flatness of the substrate. To ensure reliable results, the preparer needs to be calibrated regularly, usually using a standard thickness gauge or optical thickness gauge to verify the groove depth of the prep itself. For critical tests, it is recommended to prepare multiple wet film samples in parallel on the same substrate to assess consistency.
Applications:
This method is widely used in product development and quality control in coatings, inks, adhesives and other industries. Relevant domestic and foreign standards stipulate the specific conditions for the preparation of wet films, providing a unified basis for the comparison of test results.
| Standard number | Standard Name (Field) |
| GB/T 1727 | General preparation method of paint film (coating) |
| ASTM D823 | Standard practice for preparing uniform paint films |
| ISO 15184 | Colored paints and varnishes - use a pencil to determine the hardness of the paint film |
Notes:
When using wet film preparers, be aware of their limitations. This method is suitable for flat, hard substrates, and the uniformity of the resulting wet film thickness decreases for rough or porous surfaces. Highly thixotropic or quick-drying coatings can be difficult to achieve the desired film layer with clear edges. In addition, the thickness of the wet film is only a theoretical value, and the evaluation of the final dry film performance should be based on the measured dry film thickness. Operators are trained to master the technique of stable scraping.
Cited Literature
1. National standard GB/T 1727, general preparation method of paint film.
2. ASTM International, ASTM D823, Standard Practices for Producing Films of Uniform Thickness of Paint, Varnish, and Related Products on Test Panels.
3. International Organization for Standardization, ISO 15184, Paints and varnishes — Determination of film hardness by pencil test.