We use 2 sets of buckets to wash your car. One set is used to wash just the wheels and the wheel wells. The other set is for washing the entire car.  This prevents contaminates from the wheels from scratching the car.  

The two bucket car wash method greatly improves washing efficiency and helps to reduce damage that improper washing can cause. More times than not when people set out to wash their car, they only use one bucket filled with soap and one wash mitt or sponge. This seems practical until you realize it’s doing more harm than good to your vehicles paint.

The problem with the one bucket setup is when you go to wash your car you are trapping the dirt and grit that’s on your car into your wash mitt or sponge and that dirt and grit is then transferred into your “clean” wash bucket. Now, your “clean” wash bucket has dirt and grit particles floating around in the bucket and these particles are staying trapped in your wash mitt. Then being transferred back onto your car’s paint when you go to wash the next section, leaving many little scratches everywhere you drag your wash mitt. To minimize and avoid dirt being trapped in your wash mitt and contaminating your clean wash bucket, a second rise bucket should be used. Hence, the two bucket method was born.

The two bucket car wash method effectively removes dirt from your wash mitt and keeps it in your rise bucket. The most effective way to minimize damage to your car’s paint while washing is to wash a panel or section at a time. Once you are done washing a particular section place your wash mitt in the rise bucket, and then into your wash bucket.

To even further minimize scratches and swirls it’s a good idea to use two different wash mitts, one for above the belt line of the car and one for below. Below the cars belt line is usually much dirtier than above and it will help to minimize trapped dirt and grit coming back in contact with your paint. It’s helpful to label one wash mitt on the cuff with a star using a permanent marker to keep them separate. It’s also important to wash in a linear motion and not to scrub the car in a circular pattern, which will leave lots of noticeable swirls and scratches if any dirt or grit is stuck on the wash mitt.

Source: Incredibly Detailed