They are abrasive. If you want to get your car nice and clean use the presoak a couple times and then towel it dry. The brushes get used by everybody and while they aren't meant to be scratchless after a while they will still scratch your car a bit..
Remember if you presoak your car to much you are also go to remove your wax as well. They to washing your car is not the wash but I feel the drying of it..