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    July 28, 2020 4 min read

    Is Soft Water Good For Washing Cars?

    car wash water spots

    Not necessarily. Let us explain why - In general, there are three types of water used for washing cars: Hard water, Soft water and Reverse osmosis (RO water). People annoyed by water spots may be wary to wash their cards with hard water, as they've seen the havoc it reeks on household appliances. However, hardness represents only a part of the mineral content of water—the calcium and magnesium ions. Total mineral content, referred to in water treatment as TDS, or “Total Dissolved Solids,” is the real predictor of water's spotting potential. The higher the TDS, the more spotting you'll experience.

    Car Washing with Hard water

    water spots on car

    Hardness in water refers to its calcium and magnesium content. Hard water creates cleaning and rinsing problems when used with detergents and presoaks by consuming or reacting with some components in detergents. As a result, the amount of soap used might need to be increased to clean a car properly. In addition, there are chemical components in detergents that can react with untreated water hardness minerals to cause other problems.

    Despite all the problems with hard water in the “wash” part of a carwash, waxes and drying agents work better in hard water. The chemical components of wax or drying agent compositions will not react with the components of hard water. Hard water has the highest surface tension of the three water types, and as a result, allows for the easiest removal. With no adhesion or contact to the surface, the droplets are easy to blow off.

    When most car owners notice spotting after a car wash, "hardness" is usually to blame for the spots, leading to the assumption that a water softener will produce a spot-free wash.

    Washing a car with Soft water

    So, does soft water clean better than hard water? The answer is, it depends. A water softener may not remove the minerals causing spots out of the water. Rather, a water softener exchanges sodium for the hardness minerals, calcium and magnesium ions. The TDS of softened water is essentially the same as that of hard water. Softened water has less calcium but more sodium. Softening is an advantage to the car washer since the minerals in the softened water are easier to get rid of and don't form the scaling associated with hardness, but high TDS water, softened or not, will produce spots. With softened high TDS water, you'll still have spots. They'll just be easier to wipe off.

    However, unlike hardness ions, these sodium ions do not have any negative effect on detergents and presoaks. Soft water is the best choice to use with detergents and presoaks, as you'll be able to use less soap and still get a clean rinse. As discussed above, waxes work better in hard water so there is no reason to counteract this process with soft water.

    How Hard Water Spots Form?

    washing a car with reverse osmosis waterHard water contains minerals that end up as components of the wax/rinse droplets left on the car. As the water in the droplets evaporates, the minerals stay behind and cause white spots. Car washing spots occur when the water evaporates and any minerals in the water are left behind as residue.

    This spotting problem will still persist even if softened water is used during the waxing, but the sodium deposits in the softened water spots are easier to remove after the carwash process.

    The spotting problem is one that created the need for a Reverse Osmosis (RO) rinse. As we've explained before a water softener exchanges sodium ions for mineral ions. The softened water is then processed through an RO unit to remove the sodium salts leaving the water essentially pure. This water is ideal for the final rinse as the RO water displaces the wax/rinse water droplets and leaves drops with no mineral or wax content. Unrinsed droplets will eventually dry without leaving spots.

    Does soft water or hard water clean better?

    One thing that softened water does help with in the car wash is soap consumption. Washing your car with softened water is worthwhile is using less soap and reducing the work of wiping away spots is important to you as a car owner. best water to wash a car with, actually, is water low in TDS, such that a Reverse Osmosis will produce. Many professional car wash locations feature a ”spot free rinse“ with reverse osmosis water. Reverse osmosis reduces the TDS of water by about 95% and does assure a spot-free rinse with most waters, so that's something to keep in mind if you're looking for a cleaner rinse.


    It’s important to understand what is in your water. If you don't know the hardness or TDS level of your water, you can find out by contacting Aquatell. If you’re picky about spots on your car then make sure the water you’re using is not considered ‘hard water.’ You’ll drive yourself crazy figuring out how to sort out the water spots problem if you continue to use the same water.