IF you must go to a do-it-yourself car wash, don't use the brush. It is terriblely abrasive, and is generally designed and used to remove mud from a truck's rocker panel. Take a bucket with you filled with all your washing needs (soap, wash mit, chamois, tire wet, etc.) The following tips should help....
Always wash your car in the shade and keep it wet the entire time to prevent water spotting (I ususally spray my car 8-10 times in between rinsing just to keep it wet)
Use a wash mit, or something very dense (I'm not big on most sponges) if not sand and bits will inbed in it and could scratch your car.
I am a huge fan of Zymol products and use their wash, after wash, and wax
Start from the top and work your way down.
Use something different to clean your wheels, tires, and fender wells so you don't jack up your wash mit.
Buy an Absorber (synthetic chanmois) to dry the car off, then take it around the block to get the water out of the cracks.
Use Zymol (or your preference) spray detailer and work in with a micrfiber towel (purchased at autozone).
This should get you off to a good start.
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