Ordinary tap water may contain minerals. If we use the tap water to clean the car these minerals can end up on the paint surface. This will appear as white streaks, spots or other patterns on the paintwork, sometimes known as hard water spots. These water spots are sometime more noticeable on windows.
Tap water that contains a lot of minerals, usually limestone, calcium or magnesium carbonates, is known as hard water. As well as leaving marks on paintwork this water will not work well with ordinary detergents, not forming suds or lather. Some specialist cleaners are designed to work with hard water. But for small cleaning jobs it is probably better to use distilled water when cleaning cars and other polished surfaces.
The minerals of hard water can sometimes be removed with boiling, sometimes with other methods such as filtration. Else, it is possible to ‘soften’ the water with the right cleaners, which prevents the minerals from being deposited on the car’s paintwork.
Hand car wash will use quality detergents and filtered water to prevent minerals being deposited on and reacting with the car paint.
Hard water is generally safe for drinking. The mineral content of the water is actually part of a normal, healthy diet.