What's up with car wash "spot-free rinse"?

I have been to a number of different self-service car washes that have an option called “spot-Free rinse.” I figured that maybe they added some kind of anti-spotting agent, much like Jet-Dry. However, the main difference I see is that instead of the water blasting out of the hose, it comes out in a gentle mist that takes a lot longer to cover the car. Is the “spot-free” aspect actually the wimpy rinse, some agent added to the water, or both? Or is it just a gimmick to get people to spend more money at the car wash? If it’s the lack of pressure in the rinse, exactly how would that cause fewer spots?

For the record, I highway dry my car (IOW, I don’t dry it by hand.), and I don’t see any difference in the end results of using the “spot free” option as opposed to not using it. Just wondering what the straight dope is here.


I thought it might be filtered water; “spotting” is from dissolved minerals in normal water being deposited on the surface after the water evaporates.

I have come to the conclusion, whether it be in a self-service car wash or an automated car wash, the “spot-free” rinse cycle doesn’t even hold a universal meaning. So ultimately it’s meaningless. Some places it’s just a weak rinse at the end, some places it’s just a high-powered rinse at the end after the wax and conditioners have been applied. I’ve never known it to be a rinse with any sort of ingredients added. I could be simply uninformed, however. I’ve basically come to recognize it as a final rinse cycle, and leave it at that.

Around here, “spot free” rinses are avail with purchase of deluxe wash (top of the line, cost wise) and feature as advertised Rain-X, like the Shell stations around here. Other car washes will use a different brand of hydrophobic surface-applied product that causes water to bead, applied at the end of the normal rinse cycle.

Unfortunately road moisture often contains small beads of oil, doesn’t do that well with the Rain-X product, and some say it makes the windshield and car dirty faster than if it didn’t have the hydrophobic coating.

I think this is quite accurate. The water is highly filtered, minerals are removed and spotting is almost non-existent. I use softened water and the spotting is greatly reduced because of the elimination of minerals.

OK, so it looks like it might be something special about the water, whether it’s filtration or additives. But why the misty as opposed to blasting that water out? How would that help prevent spotting?

cite? (that it is simply filtered water used in “spot free” car wash rinses)

They’ve already done the rinsing - now they just need to get the anti-spot stuff on the car. Misting allows them to do this while using less of it. If they just pour it on, most will run right down the drain.

I’m sorry, I figured that my use of the phrase “I thought it might be filtered water” would preclude the usual braying for a cite. Silly me.

Just a nit to be picked or a curiosity for those who may be interested, the water filters are often chemical, not physical, ones. A “physical” filter acts like a sieve, it’s got holes where the removed stuff gets stuck simply because of size (active carbon and diatomaceous earths are the most extreme examples of this); a “chemical” filter, such as a water ion-exchange purification system, contains an inert polymeric core with lots of reactive branches, and the ends of these branches get exchanged chemically with the items that need to be removed.

In the case of a water ion-exchange purification system, there is a part of the system which removes negative ions (such as Cl-) and provides hydroxils (OH-), another which removes positive ions (Na+, Ca[sub]2[/sub]+, Mg[sub]2[/sub]+).

Exchange columns are used in many other applications, mentioning just two for info’s sake:

  • removing lactose from lactose-free milk (depending on the location, the end result has legally to be called “lactose-free dairy product”) in a way that doesn’t leave the milk tasting sweeter than regular milk (adding lactase to break up the lactose without removing it, while perfectly fine from a health point of view, breaks the lactose into two smaller, sweeter-tasting sugars… this leads to consumer rejection)
  • synthesizing (small) proteins, which IIRC correctly was the original use of this kind of system

A carbon active or DE filter can remove tiny particles, but not salts.

And just as anecdata, the “distilled water” which was so popular for carwashing in several of the factories where I’ve worked went through DE or carbon / ion exchange / DE or carbon, to remove: particulates, salts, any possible particulates broken off from the ion exchange.

Cut jz78817 some slack.

There’s this product that guarantees “spot free” washes.


The “spot free” claim is based on the filters, not the washing agent. I’ve used one. It only makes sense that a commercial “spot free” wash is using some similar system on a larger scale. The Mr. Clean system also uses a fine mist spray on the rinse cycle when the water is being filtered. I don’t know but it may be that the filters are so fine that only a limited amount of flow can be realized. If so, turning it into a fine spray may make it more effective.

There may be “spot free” products avail from Amazon or other retailers but the question was concerning commercial spot free rinses at car washes.

Spot free car wash rinses typically contain a hydrophobic silicone polymer product - a synthetic hydrophobic surface-applied product that causes water to bead. Like Rain-X brand which is advertised by many car washes (like Shell gas station washes, Rain-X is subsidiary of Royal Dutch Shell) as the “spot free” rinse additive.

The stuff indeed repels water, you can see it working on the windshield when it rains, and often for several rains after receiving a single “spot free” rinse - one can still see it working by causing the water to bead.


Well I don’t know if you’re silly but I think the forum you’re looking for is In My Humble Opinion.

I think you are looking for the PIT but, never-mind.

Its water that goes through a reverse-osmosis process and is stored in a holding tank. It contains less than 10ppm (parts per million), which means it leaves no residue after it evaporates. Its applied at lower pressures to prevent wasting the water. It is very expensive water to blast out at high pressure.