Study pictures of dirty cars on the Internet. This will give you a realistic idea of the way that dirt behaves on a truck or a car. Study where dirt gathers, what color it is and whether you can see the color of the car beneath it.
Prime the surface of the car with a spray primer.
Choose a color for your car and shake the paint well. Mix a drop of acrylic extender on the palette with the paint for the car. Paint the surface of the model car.
Mix a drop of acrylic extender on the palette with the color of the dirt to be used. Light brown, medium brown or dark brown are all appropriate colors. If you have in mind a specific type of geography for the model car, consider the color of the landscape before choosing the color of the dirt. For example, Georgia soil will have a much redder tint, while sand from the Mojave desert will appear to be a light brown-yellow.
Use a fine paintbrush to apply the dirt. Most dirt on a truck or car will come from splatter from the wheels. Therefore, most of the dirt will be located in back of the wheels in a diagonal line that moves upward. Before the base coat of the car or truck has dried, paint a triangle of dirt that begins at the back base of the wheel and fades as it moves upward. Allow the base color of the car to mix with the color of the dirt, which should be slightly transparent.
Splatter dirt on the areas of the truck or car behind the wheels for effect. The splatters should be tiny, and there should not be too many of them. To splatter the paint, quickly flick the paintbrush at the intended target. Practice before splattering paint on the model car.