How to Apply Protective Coatings in Automotive

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Protective coatings play a key role in the automotive industry – they protect metal and plastic components from corrosion, chemical influences, UV radiation, and mechanical damage. However, even experienced technicians often make mistakes that reduce the coating’s effectiveness and shorten the lifespan of components. Whether it’s improper surface preparation, incorrect mixing of additives, or failure to adhere to technological conditions, the consequences can be costly. How can you ensure that the protective coating fulfills its function throughout the entire lifespan of the component?

Improper Surface Preparation: The Foundation for Coating Failure

Surface preparation is a critical step that often determines the lifespan of a protective coating. Many manufacturers in the automotive industry underestimate thorough degreasing, removal of oxides, or old coatings, which leads to insufficient adhesion and subsequent peeling. The surface must be not only clean but also dry and free of dust—even microscopic particles can create a barrier between the material and the coating. To achieve optimal results, a combination of mechanical cleaning (e.g., sandblasting) and chemical degreasing with suitable solvents or alkaline cleaners is recommended.

Another common mistake is neglecting surface treatment before applying the coating. For metal parts, it is essential to ensure passivation or phosphating, which improve corrosion resistance and adhesion. For plastic components, surface activation with plasma or special primers is often overlooked. Without these steps, the coating may crack or peel over time, especially when exposed to temperature fluctuations or mechanical stress, which is common in the automotive industry.

Improper Storage Conditions: How to Degrade Raw Materials Before Use

Storing coating materials and additives requires careful attention, as improper conditions can significantly reduce their effectiveness. Temperature and humidity are key factors—most coating systems should be stored at temperatures between 10–25 °C and relative humidity below 60%. Exceeding these values can lead to premature curing, sedimentation, or degradation of active components. For example, some polyols or coating additives are sensitive to moisture and can absorb water from the air, which deteriorates their properties.

Another issue is the expiration of raw materials. Many manufacturers do not record the opening date of packages or do not manage stock rotation according to the FIFO (First In, First Out) principle. Old coating materials may lose viscosity, homogeneity, or reactivity, leading to uneven application and reduced durability. It is recommended to regularly check inventory and adhere to the recommended shelf lives provided by manufacturers in technical data sheets.

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Application Errors: Technology and the Human Factor

Applying protective coatings requires precise adherence to technological parameters, which are often underestimated. One of the most common mistakes is incorrect coating viscosity. A coating that is too thin may run and create an uneven layer, while a coating that is too thick spreads poorly and may contain air bubbles. Viscosity should be adjusted according to the manufacturer's recommendations, ideally using suitable thinners under controlled conditions (temperature, humidity).

Another critical point is the coating thickness. A layer that is too thin does not provide sufficient protection against corrosion or mechanical damage, while a layer that is too thick may crack or peel due to internal stress. Standard test methods allow measuring the thickness of wet and dry coatings, enabling adjustment of the application technique (spraying, dipping, roller application). Uniformity is also important – differences in thickness can lead to local coating failure.

Insufficient Curing and Quality Control: Hidden Defects with Long-Term Consequences

Curing the coating is a process that cannot be rushed. Many manufacturers underestimate the time required for complete drying and curing, leading to insufficient mechanical strength or chemical resistance. Temperature and curing time must comply with the manufacturer's specifications – for example, some epoxy coatings require temperatures above 60 °C for several hours. Failure to meet these conditions may result in the coating remaining soft or brittle, shortening its lifespan.

Quality control is the final but no less important step. Visual inspections (bubbles, cracks, unevenness) or adhesion tests (e.g., cross-cut test) are often overlooked. Without these checks, defects may only be detected during operational stress, leading to costly repairs or complaints. It is recommended to implement regular inspections according to applicable standards and document the results to ensure consistent quality.

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Failure to adapt the coating system to operating conditions

In the automotive industry, the impact of operating conditions on the selection of a suitable coating system is often underestimated. For example, parts exposed to high temperatures (engine compartments, exhaust systems) require coatings with heat resistance above 200 °C, while components in the vehicle interior may prioritise abrasion resistance or UV resistance. A mistake is to use a universal coating regardless of specific requirements – for instance, epoxy systems excel in chemical resistance but degrade and lose colour when exposed to sunlight over the long term. Similarly, coatings for parts in contact with fuels or oils must be compatible with these media; otherwise, swelling, cracking, or delamination may occur.

The solution is a thorough analysis of operating conditions before selecting raw materials. It is recommended to consult with a chemical raw materials supplier, who can recommend additives to enhance resistance (e.g., UV stabilisers for exterior parts) or special resins for extreme temperatures. It is also important to consider mechanical resistance requirements – for example, coatings on the underbody must withstand abrasive wear from gravel, which requires flexible yet hard polymer systems. Testing samples under simulated conditions (e.g., cyclic temperature tests, chemical exposure) helps prevent premature coating failure.

Ignoring Compatibility Between Coating System Layers

Multi-layer coating systems are standard in the automotive industry, but their effectiveness depends on the compatibility of individual layers. A common mistake is combining incompatible resins or additives, which leads to layer separation, blistering, or insufficient adhesion. For example, acrylic bases may react poorly with certain polyurethane topcoats if not formulated correctly. Similarly, using an unsuitable primer under an epoxy layer can cause a chemical reaction leading to cracking or loss of elasticity.

The key to success is adhering to the technical specifications provided by raw material manufacturers and conducting compatibility tests before full-scale production. It is recommended to test adhesion using the cross-cut test or grid test according to applicable standards. It is also important to consider the drying time between individual layers – applying the next layer too quickly can trap solvents and subsequently lead to pore formation. In case of doubt, it is advisable to consult the raw material supplier, who can recommend optimized systems with verified compatibility.

Underestimating the Impact of the Environment on Coating Application

The application environment has a significant impact on the quality and durability of the coating, yet it is often underestimated. For example, high air humidity (> 70%) can cause water condensation on the part's surface, leading to bubble formation or dull spots in the coating. Similarly, the temperature in the application booth must be within the optimal range (usually 18–25 °C) – too low a temperature slows down curing, while too high a temperature can cause premature solvent evaporation and surface defects. Dust and dirt in the air can become trapped in the wet coating, creating visible defects.

To minimize these risks, it is essential to monitor and regulate the conditions in the application booth. It is recommended to use air conditioning systems with air filtration and humidity and temperature monitoring. Before application, it is advisable to wipe the part's surface with an antistatic cloth to remove any dust particles. For water-based coatings, it is particularly important to maintain low humidity to prevent uneven drying. When applying outdoors, it is necessary to choose suitable weather conditions and protect the parts from direct sunlight or rain during drying.

Need advice on selection or application?

Every chemical raw material requires a specific approach – from proper dilution to adherence to technological parameters. GCG Group provides detailed safety data sheets and technical data sheets for all products, and our experts will be happy to advise you on selection and application for your specific production. Contact us – or browse our catalog of over 1,300 products right away.