When you pressure wash (or touchless car wash) a vehicle, there is a thin, almost invisible layer of dirt that will remain that you can see if you wipe a finger across the just washed surface. What keeps that thin layer of dirt on the surface under the high pressure of a wash like that?
In: Chemistry
Some trees release this glue-like sugary stuff, that basically sticks to your car and keeps the dirt there. This is why car washes with brushes are still preferred by some, and why manual car washing still happens. The water needs to sit on the surface to soften up and dissolve that layer of sugary stuff, and scrubbing with a sponge helps take it off.
A touchless car wash only removes the loosest dirt particles. The really dried-on, stuck-on particles have to be manually scrubbed off.
It’s just like with your dirty dishes. If you try to wash them with just a squirt of soap and the sprayer attachment, you’ll get a lot of the crud off, but you’ll never get all of it unless you use a sponge or scouring pad.
I managed a car wash for many years and it’s commonly called not scrubbing your car. Depending on what part of the world you live in it could also be what ever it is they treat the road with during the colder seasons. Your best bet is to fully clean your car properly then wax it, the wax will help road grim not stick to the car making the car wash work better at removing the film. if you can’t manage to remove the film prior to waxing you should buff it out using a low power buffer unless you don’t mind braking a sweat.
It’s a fluid dynamics phenomenon called the no-slip condition. When water flows through a pipe or along a riverbed, the water is always moving slower the closer it gets to the walls. Even in a fast-moving river, the water barely moves at the edges which allows for microbial growth, sediment and other buildup.
This could work similarly with a pressure washer.
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