Do it yourself car washing soap made from vegetable oil based soap is indeed safe for use on your car’s exterior. In fact, when making your do-it-yourself car soap, vegetable oil based soaps are a good option to consider. Vegetable oil based soaps are generally made without phosphates or chlorine, which not only makes them safer on the environment, but also safer on your vehicle’s paint. When making your own car soap with vegetable based soaps, the soap should simply be mixed with warm water in a mixing container such as a bucket. From there, use as you would any car washing soap.
For more info, read How to Make Your Own Car Wash Soap