Dawn is gentle enough to use on most car finishes, so you don’t need to worry about scratching or damaging the paint. However, there are a few things to remember if you use Dawn to wash your car. Be sure to use a diluted solution of Dawn and water. Too much detergent can strip away wax and protective coatings. Rinse your car thoroughly after washing it with Dawn. Dish detergent can leave behind a streaky residue if it's not rinsed away properly.  Avoid washing your car in direct sunlight. The heat can cause the soap to dry too quickly, leaving behind water spots.