Spots are usually caused by water.the water when dried leaves a white spot which are unwanted.these are forms of mineral deposits that the water contained.that is why I use filter when I wash my Mercedes Benz especially on its body and also in its interior like Mercedes Benz cargo liner which I find it unattractive to see those spots in my car.Here in Florida is a country which has large amount of mineral deposits in water.so I make it sure that I filter the water from the faucet before I use it to clean my car.