What causes water spots

Water spots are caused by desolved minerals in water. Calcium carbonate (sometimes called lime) is the most common and appears white and chalky when dry. These minerals form ions when desolved in water; when the water evaporates, it leaves the minerals behind. That's why water heaters, coffee pots, and humidifiers get coated with minerals over time. The harder your water, the more desolved minerals.

When you rinse your car and let it air dry, the water evaporates and leaves behind minerals, otherwise know as water spots. They form spots because water tends to pull together as droplets, and as each droplet evaporates, it leaves behind a little white coating of mineral.

Given this information, we see that there are several ways to treat this problem:

1) Remove the water before it evaporates. If you dry a surface with an absorbent material, the minerals are absorbed with the water, so no water spots occur. You can also remove the water with a squeegee or chamois. In either of these cases, the goal is to remove as much water as possible before evaporation occurs. The less evaporation, the less mineral to leave spots. This is why you get more spots washing in the sun - the heat speeds evaporation.

2) Use a wetting agent to prevent droplets from forming. The wetting agent reduces the surface tension of the water, so the water does not pull together to form droplets. This helps the water run off the surface, much like using a squeegee or chomois. The remaining water still contains minerals which are left when it evaporates, but the minerals are deposited evenly over the surface instead of being consentrated as spots. The small, light coating is much less objectionable than spots.

3) Use water containing few minerals. If you rinse a car with deionized water, you can just let it air dry. Since the water does not contain minerals, no spots will be left when the water evaporates. This is by far the easiest and most elegant solution to water spots, but you need to find a way to get deionized water. Alas, there is no free lunch.

How to deionize water:

1) Soft water, as most of us know it, is not even remotely deionized. Chemical water softeners work by trading hard water ions (calcium) for another type of ion (usually sodium). The treated water has just as many ions, but the new ions are a type that interferes less with soap and sudsing, and are less likely to to build up on faucets and fixtures. This is the premise behind water softeners that use salt, and water softening addatives such as washing soda and lemon juice. These all help reduce water spots, but are not completely effective.

2) Commercial water treatment plants often soften water by treating large batches with chemicals that cause the hard water ions to clump together and fall to the bottom of the treatment pond. This is called chemical precipitation. If you have city water and it is treated this way, you are going to have a lot less trouble with water spots. Consider yourself lucky. Otherwise, this is not something you are going to do at home.

3) Hard water ions can be removed by mechanical filtration through reverse osmosis. A reverse osmosis filter squeezes water through a very fine membrane that allows water to pass but catches most of the slightly larger hard water ions. Reverse osmosis will remove many, but not all, of the hard water ions. For many applications, this may be good enough.

4) Distillation. Distillation does not technically produce deionized water. Distilled water still contains many ions from the distillation plumbing and tankage, but for the case in point, the nasty calcium ions are left behind at the still. Essentially, destilled water has already been evaporated, so the distiller gets the calcuim and your car skips the spots.

5) Deionizing filters. Deionizing filters work by using a two step process where ions are exchanged and then trapped in a filter bed. Deionizing filters can make water so pure that it will pull the copper right out of your pipes. Obviously we don't need lab grade deionized water for washing a car - an industrial grade filter will work just fine. Commercial systems that use large amounts of deionized water usually start with an ion exchange water softener, then feed it to a reverse osmosis filter to remove most of the ions, then finish off with a deionizing filter. This may be overkill for your average car washer.

Here is my recommendation:

After washing your car, rinse with deionized water. Distilled water will work fine, but it is expensive and hard to use since in comes in bottles. Your best bet for deionized water is a system such as Mr. Clean's. The small deionizing filters are expensive per gallon compared to a commercial filter, but are pretty cheap to buy since they are small, plus they are convenient to use. You won't get a high flow rate through a small deionizing filter, but you shouldn't be using deionized water to blast bugs off the bumper. You want to do all of the washing and blasting before your final rinse. The deionzied water is only used as a final rinse to remove all of those nasty calcium ions, then you can just let your car air dry.

If any of you car wash nuts out there want to get serious about deionization, contact me and I can give you some sorces for larger deionization filters.

Hope this helps,
Tom