Recently there was a Tech Article in an RV publication about cleaning the tires on an RV. The reply reminded me of some investigations I did a number of years ago on some strange looking sidewall damage. After doing a confirming test, where I was able to duplicate the damage, I concluded that the damage was done by a High Pressure Power Washer.
Here are some examples of the damage. You can click on the image to see an enlarged view.
Another example is seen here:
In sample three you can see the cut that was made to confirm the depth of the damage.
I have never seen a advisory posted on one of these High Pressure Power Washers warning of the damage that can be done if you get too close. There are many different pressure ratings so I cannot give an exact limit on how close you can get without doing damage but I would certainly stay more than a foot away if I felt I needed to use one.
This damage has been documented in Tire Industry publications so if you do cause this type of damage, there is a good chance you may not be compensated under the tire warranty, so be careful. The best way to clean your tires is to use the same soap you use to clean your car or RV and a sponge or cloth just as you would on the paint on your vehicle. I would not use a stiff brush or anything like steel wool on the tire as you will scrub off the protestant waxes built into the tire which will accelerate the tire sidewall aging and could promote sidewall cracks. I would limit the pressure I use to what I get out of a garden hose.