Where the streaks on a clean car come from
A familiar situation: the car has been washed carefully, the foam rinsed off, no dirt left — and ten minutes later whitish patches and streaks appear on the body. It looks as if the wash failed to rinse the chemistry off. Almost always the chemistry has nothing to do with it.
Any tap water has salts dissolved in it — calcium and magnesium above all. While the water is on the body you cannot see them. The water evaporates, but the salts cannot: they stay on the clear coat exactly where the drop stood. Hence the characteristic pattern — the outlines of drops and runs.
The harder the water in the city, the more noticeable the effect. And that is exactly why the same wash in two different cities gives different results with the same care taken.
What hardness is and how to find yours
Hardness is precisely the amount of dissolved calcium and magnesium salts. It is measured in degrees of hardness, and the spread across the country is very wide: in some places the water is naturally soft, in others the well water is such that a kettle furs up in a month.
There are three ways to find out your water hardness. Look at the city water utility's website — they publish figures by district. Send a sample to a laboratory — it is inexpensive. Or buy a home test kit.
For the owner of a bay this is not an idle question: hardness determines the choice of shampoo, how much of it you use, and whether water treatment will be needed. In hard water part of the active ingredients goes not on the dirt but on binding the salts — that is, you are paying for chemistry that never reached the bodywork. More about this in the article about car care chemistry.
Water hardness has no effect on drying wax — the product data sheets state this directly. Unlike shampoo, it works the same in soft and in heavy water.
What osmosis water does — and what it does not
Reverse osmosis is filtration through a membrane with very fine pores. Water is forced through it under pressure, while the dissolved salts stay on the other side and go to the drain. What comes out is water with virtually no salts in it.
Hence the whole point of the final programme in the bay: water without salts dries leaving no marks. There is nothing to evaporate except the water itself.
What osmosis water does not do is clean. It is a rinse, not a wash. Run it over a half-washed car and you are simply pouring expensive water over the dirt. It comes last, once everything else is done.
And one more thing: osmosis water does not replace drying, it makes drying easier. The body still has to dry — with the blower or simply with time.
Why you must not reach for a cloth after osmosis water
The most galling mistake at the very last step. Someone takes the osmosis rinse, pays for it — and then pulls out a cloth to “finish the job”.
A cloth puts back onto the clear coat exactly what you have just got rid of. It carries dust from last time, and together with the remaining water it drags across the body the grains of sand that always stay behind after a wash. The result is a web of fine scratches, clearly visible in the sun and on dark cars.
The right finish: osmosis water, then the air blower. Air dries without touching the body. If the bay has no blower, let the car stand for a couple of minutes: after osmosis water it dries by itself and without marks. That was the point of the programme.
About pressure: why more does not mean cleaner
The second question, asked almost as often: what pressure the bay has and whether it is enough.
The exact figure depends on the machine, but it has almost no practical significance — because what reaches the bodywork is not the pressure at the pump but the pressure at a distance. The jet loses force very quickly, and the difference between “holding it at twenty centimetres” and “holding it at forty” is bigger than the difference between two different machines.
The working distance in a bay is about twenty-five to thirty centimetres. Closer is unnecessary and risky: a point-blank jet lifts seals, peels back the edge of a film, knocks off stickers and finds the weak spots in old paint. Beyond forty centimetres it stops making sense — the dirt no longer comes off.
And the main thing worth understanding: touchless washing cleans with chemistry and time, not with pressure. Pressure is there to knock the sand off at the start and rinse the softened dirt away at the end. Everything else is done by the foam while you wait one to three minutes. Skip that pause and no amount of pressure makes up for it — you will only spoil the result.
Frequently asked questions
What is osmosis water at a self-service car wash?
It is a rinse with water from which the dissolved salts have been removed by a membrane. Ordinary water leaves whitish streaks as it dries, because the calcium and magnesium salts stay on the clear coat. Osmosis water contains virtually none of them, so it dries without a trace. Osmosis water does not clean — it comes last, once everything else is done.
Why are there streaks after a wash?
Most often it is not leftover chemistry but salts from the water. While the water is on the body you cannot see them; the water evaporates and the salts stay exactly where the drop stood — hence the characteristic outlines and runs. The harder the water in your city, the more noticeable the effect. Two things help: a final rinse with osmosis water and drying with the air blower instead of a cloth.
Should you wipe the car after osmosis water?
No, and that is the whole point of the programme. A cloth puts back the dust it carries and drags the remaining grains of sand across the body — on dark cars that turns into a web of fine scratches. After osmosis water the body is dried with the air blower or simply left to dry: no marks will remain.
How do you find out the water hardness in your city?
Three ways. Look at the city water utility's website — figures by district are usually published. Send a sample to a laboratory, which is inexpensive. Or buy a home test kit. For the owner of a bay this matters: hardness determines the choice of shampoo, how much of it you use and whether water treatment is needed.
What pressure do you need at a self-service car wash?
The exact figure depends on the machine, but in practice the distance matters more: the jet loses force very quickly, and the difference between twenty and forty centimetres is bigger than the difference between two different machines. The working distance is about twenty-five to thirty centimetres. Touchless washing cleans with chemistry and time, not with pressure.