What causes shampoo residue or streaks after washing a car?

Common causes include using too much shampoo, allowing it to dry on the paint, poor rinsing, hard water and washing hot panels. Residue can also appear when concentrated product remains in crevices, gaps and trim and runs out after drying. Follow the dilution on the label, rinse each section before it dries, rinse thoroughly from top to bottom and dry the car promptly.