“Hardness” in water refers to its calcium and magnesium content. “Softening” in the conventional sense means removing these minerals by exchanging sodium or potassium for them.
When spotting occurs after a car is washed, “hardness” is usually blamed for the spots. This leads to the assumption that “softening” the water will produce a spot-free wash.
Not necessarily so. 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.
Softening, contrary to popular belief, doesn't take minerals out of the water. Rather, it exchanges sodium for the hardness minerals, calcium and magnesium, in more or less equal proportions. 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 spotting. With softened high TDS water you'll still have spots. They'll just be easier to wipe off.
Car washing spots occur because the water evaporates and any minerals in the water are left behind as residue.
The best car wash water, of course, is low TDS water. Removing all of the mineral content of water, however, is not practical in most cases. The process of deionization removes all the minerals and will certainly produce a spot-free wash, but it’s impractical because of the expense for the home user. The same is true of distillation. 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… that’s something to think about if you're a hard-core car washer.
One thing that softened water does help with in the car wash is soap consumption. If using less soap and reducing the work of wiping away spots are important, washing your car with softened water is probably worthwhile.
If you don't know the hardness or TDS level of your water, you can usually find out by calling your local water supplier.
(whole house & well units)