Powder coating adhesion: what to do when paint does not adhere

Powder coating is one of the most popular processes in industry for achieving strong, durable finishes with a strong aesthetic impact. However, even the most experienced professionals face a frequent defect: failure of the paint to adhere to the substrate.

This paint defect can have several manifestations:

• the coating does not adhere to the substrate in any way;
• the paint adheres to the piece only partially;
• the coating does not reach some parts of the surface;
• paint adheres, but does not last over time.

To understand the extent of the problem, it is useful to measure the degree of adhesion. The degree of adhesion can be measured by a “cross-cut” or “cross-cut” test.(ISO 2409:2020 – Cross-cut test).

It is a simple but standardized method for objectively identifying any defects before the part moves on to the next stage of processing. The test result is graded according to the scale provided by ISO 2409, providing a clear and verifiable figure that is useful both for internal quality control and for comparing with suppliers or those who handle pretreatment and painting.

So what is the cause of powder coating adhesion problems?

It is not easy to identify just one, as the powder coating process is subject to both painter error and surface characteristics of the part, two factors that can overlap and complicate analysis on the origin of the defect.

In this article we are going to address the main issues of powder coating adhesion and the possible solutions to be implemented to achieve a long-lasting result that can withstand external agents. In particular, we will focus on how to solve the lack of adhesion of powder coating by:

• The optimization of pretreatment to remove impurities;
• The correct calibration of plant parameters;
• The management of complex geometries.

Before painting, debris left on the part from previous processing and any contaminants must be removed. To remove these, mechanical and/or chemical treatments are required. Depending on the material to be painted, surface cleaning can be done mechanically by sanding, sandblasting, or brushing. These treatments use abrasive action to remove both structural defects and previous coatings from the part.

Next, each part must be cleaned of dirt and grease residues, using detergents and solutions specific to the substrate and type of contaminant. In the industrial setting, the washing phase can be carried out using automatic multi-step tunnels. These systems, such as those that Eurotherm has developed through years of experience, make it possible to customize part preparation according to the technical specifications of the material and its future application.

Pretreatment must be calibrated according to the intended use of the item and the required performance. Powder coating also finds application in the ACE (Agricultural, Construction and Earthmoving equipment) sector, which includes agricultural, construction and earthmoving equipment, where mechanical strength, corrosion protection and durability of the coating are essential.

The electrostatic charge of the workpiece and the firing temperature are the two key elements to be controlled to ensure proper adhesion of the coating.

How to properly arrange parts to ensure electrostatic charge and dust adhesion

During paint application, workpieces must be electrostatically charged in order for the powder to adhere to the surface. If instead of settling on the workpiece, the paint settles on the operator, the electrostatic charge may not have been transmitted to the workpiece to be coated.

To solve this problem, it is necessary to check that the flight bars that support the piece are clean. The paint residue acts as an insulator and does not allow the charge to be transported, thus preventing the dust from adhering to the surface. These small steps are part of plant maintenance. If not done carefully, the entire production will suffer.

This phenomenon, known as the Faraday cage effect, leads to uncovered or poorly coated areas, with negative consequences on aesthetics and strength. On our website you can learn more about how Eurotherm designs powder coating systems to minimize these defects in productions with corners, recesses and irregular surfaces.

Selection and maintenance of spray nozzles: precision for a uniform finish

To apply powder coating to specific areas, it is necessary to check the nozzles that will modulate the delivery from the gun. Flat spray nozzles are suitable for covering large areas, while round nozzles are suitable for recesses and small indentations. Nozzles also require maintenance. These attachments can retain residual dust from previous applications, so it is important to clean them after each job. Accumulated dust not only contaminates the coating but also prevents even and consistent paint delivery.

High-penetration coatings and epoxy resins: solutions for complex surfaces

Due to constant innovation in industrial coating, high-penetration thin-film coatings have been developed that counteract the “cage” effect and reach surface indentations. An optimal solution, but one that may not be suitable for your production.
If compatible with your needs, another option may be epoxy: a highly elastic coating with good adhesion both on linear surfaces and on corners and edges.

To improve powder coating adhesion in industrial processes, Eurotherm develops customized solutions that integrate vertical flow coating booths with filter roofs, multistage washing tunnels, and ovens calibrated to eliminate cold zones and ensure uniform cure. The goal is to ensure process continuity, waste reduction and consistent coating quality, with custom-designed facilities where powder coating is the technology of choice for strength, durability and mechanical performance.