Hold a torch to dark paint and you'll see fine circles around the light. Those are swirl marks: thousands of tiny scratches in the clear coat, and most of them were put there by a wash mitt.
Where they come from
Dirt is abrasive. Drag it across a panel and it leaves a mark. Automatic brushes, dirty mitts and dry towels are the usual causes.
A safer routine
Rinse first, so the worst of the grit leaves before you touch the paint. Use two buckets, one for rinsing the mitt. Work from the top down, and dry with a clean, soft towel or filtered air.
Once marks are there, only polishing removes them. But a careful routine means you won't need that work again for a long time.
