ZH 
EN 
For anyone who owns a car, washing it is routine. Prices vary wildly, from a few dozen yuan at a roadside wash to several hundred for a premium package. So what is really behind the difference?
Broadly there are three kinds of cleaning agent in use: laundry powder, dishwashing liquid and purpose-made car shampoo.
Yes, some washes genuinely use laundry powder or dishwashing liquid.
Laundry powder is alkaline, and plenty of washes mix their own brew: powder, detergent and water. It does real damage to paint. Wash with laundry powder long enough and the surface gradually loses its gloss. The corrosion is hard to see in the short term, which is why many owners never notice.
So why use it when the damage is known? Because it is cheap and fast. Some washes use dishwashing liquid, usually bought in bulk drums like the catering trade. It saves only a few dozen yuan over proper shampoo, yet is widely used.
Then there is labour. Many washes hire temporary staff with almost no training; some start on the job having barely touched a car.
Given how much is hidden, it pays to ask questions when you buy a wash.
From everything we have gathered, a shampoo that carries wax is the best option. When you use a wash, ask whether their shampoo contains wax. A wash-and-wax shampoo is mild, cleans well, and does not irritate or damage the paint, while adding some gloss.
In practice you see two extremes: owners who wash every other day, and owners who go months without. Neither is right. Once a week is about right for most. Light dust can be dealt with using a duster or soft mop, but after mud, grime or heavy rain, wash as soon as you can.