The sanding primer is an important part of the process when you want to achieve a smooth surface on the car before applying the paint, especially when it comes to metallic colors that are then finished with clear coat. The sanding primer is also used when replacing a panel that has a different color to the rest of the car, as it provides a neutral base color that does not affect the shade when painting the entire car. By using sanding primer, you can ensure a smooth and professional finish on your car's paintwork.