Prakt. Met. Sonderband 30 (1999) 221 
“Metallographic Preparation and Quantitative Image Analysis of Automotive 
Paint Specimens” 
Authors 
Matthias Hoffmann, Buehler Ltd., Lake Bluff, Illinois, USA 
William Creech, BMW Manufacturing Corp., Spartanburg, South Carolina, USA 
Introduction 
Typically, four different layers make up the paint on a car part such as a door panel. The integrity of 
the various layers is essential for the longevity of the overall part. Automotive paints are exposed to 
the natural elements and potential damage from flying debris eventually causing corrosion damage. 
Various tests have been established in the automotive industry to simulate the exposure of a car to 
the elements to further characterize the paint quality. Special techniques are used to prepare 
specimens metallographically to quantify the paint layer uniformity and paint flaws. The layer 
thicknesses is analyzed using automated image analysis equipment. 
Mechanical Paint Evaluation Tests 
Often, debris will damage the paint layers of the car so severely that the metal surface gets exposed. 
Over a period of time, corrosion takes place and severe damage of the metal structure may occur. 
The goal is to improve the durability and resistance of these paint layers to mechanical debris. A 
number of mechanical tests are established to simulate the actual road conditions and exposure of 
the car to the elements. Below is a brief description of common mechanical test helping a car 
manufacturer to improve the paint process and ultimately come up with more durable paint finishes 
on a car. 
» Cross-Hatch Paint Adhesion Test 
This test is designed to measure the adhesion of the paint to the steel substrate. A grid is scribed on 
the painted panel using a sharp knife. The grid width is determined by thickness of the paint. The 
grid pattern used is 6 lines by 6 lines with 2 mm distance in between lines. A specialized tape is 
then applied over the grid area and is pressed firmly on the surface to ensure even contact. The tape 
is then quickly pulled from the surface at a 60-degree angle (1). 
The grid area is then evaluated by calculating the total percentage of the small blocks in the grid that 
show delamination. The lower the percentage the better the performance of the paint. 
» Multi-Stone Test 
This test is used to determine a painted surface’s resistance to impact from gravel. 
500 grams of a steel shot material is placed in the top of the “Multi-stone” machine. 
The test part is securely placed at the end of the machine with a clamping device. The steel shot is 
vibrated down into a pipe where it is forced by compressed air (2 bar of pressure) to strike the 
painted surface of the test panel. 500 grams of steel shot must impact the test panel within 15 
seconds from the start of each run. The impact area is then covered with a specialized tape and