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Automotive paint for protection of vehicle body

The quality of automative paint influences how well it can protect the car body against environmental influences. This article explains the basics of painting a car and how rotational rheometers can be used for that.

Rust on cars

Rust on cars

Rust on cars

The quality of automotive paint is crucial for preventing rust on cars. Rust originates once an anode, a cathode, and an electrolyte are present. The bodywork of cars serves as the anode/cathode, water – such as simple rain water – can function as the electrolyte. As soon as the automotive paint levels off or gets a scratch, the protective effect of the car paint is lost and an electrochemical process occurs, leading to corrosion. Since the car body is mostly made of different kinds of steel, different iron oxides accrue, and rust becomes visible. Once the protective layer is damaged and rust forms, its progress occurs inconspicuously.

Painting a car’s body

The automotive paint is usually applied to the car body in several layers, each layer having a thickness of a few micrometers. There can be more than 20 process stages (including cleaning, degreasing, drying, etc.). These are the key stages of the process:

  • Preparation: The car body is cleaned and degreased thoroughly. A phosphate coat is applied as the first layer. It serves as a corrosion protector and later on provides the necessary layer to bond the paint.
  • This step is followed by electro-coating. Solid paint particles are deposited onto the chassis via electrophoresis.
  • A spraying process (e.g. via spray booth systems) is used to apply the primer. The primer serves as a leveling and protective layer.
  • Afterwards, the base coat is sprayed onto the car body. The final finish of the base coat can be e.g. solid or metallic. While the solid one exhibits easier processability, the metallic one is more difficult to apply to the car body since solid particles need to be aligned in one direction to obtain a smooth surface.
  • The last step is to apply the clear coat. The clear coat is used to protect the car body, which is usually made out of steel, against abrasion and UV light. Whereas most cars have a shiny, clear, reflective, and glossy appearance, a recent trend has been embraced by automotive car manufacturers – the matte finish. Matte finish is mostly applied within this process step and leads to a hazy appearance.

Rotational rheometers in the automotive paint application process

Knowing about the viscosity of the automotive paint is essential for its application process, its final quality, and long-term endurance. There are two main applications for a rotational rheometer to support in achieving the perfect processability and finish of the automotive paint:

  1. The paint is usually pumped through pipes to the spray nozzle. A rotational rheometer can help in the determination of the flow behavior of the automotive paint at different shear rates. Thus, the shape of the pipes can be ideally designed.
  2. Studying the thixotropic behavior of the automotive paint is most important for its final appearance on the car. The surface of the car should be smooth and perfect. The three-interval thixotropy test (3ITT) simulates the behavior of the automotive paint during the spraying process:
    • Low-shear interval: The behavior of the automotive paint at rest is evaluated.
    • High-shear interval: Load is applied to the sample to cause structural decomposition. This step is equal to the stress applied to the automotive coat during the spraying process.
    • Low-shear interval: How does the automotive paint regenerate and stick to the base coat? The structural recovery needs to be adjusted to guarantee optimal leveling behavior and prevent sagging.

This information is highly relevant for the whole paints and coatings industry.


The application of automotive paint on the car body is one of the most demanding process steps when it comes to painting a car. A high-quality finish is necessary to prevent its abrasion and to ensure that a uniform protective layer is formed, for example against UV light or moisture. The rotational rheometer RheolabQC helps reach perfect processability. It supports quality control steps before and during the application process – to ensure long-term corrosion protection.