Andrew - question? I always thought swirl marks were caused by fine scratches in the clear coat? I was always told to never wash/wax the car using a circular motion and always use a good car shampoo like Meguiars and make sure the car is rinsed off thoroughly before you hit it with the wash mit. Dry it with 100% cotton towels (labels and ends cut off) or microfiber towels.
I know UV rays can damage paint, I wasn't aware they caused swirl marks.
Washing a dark car in direct sunlight causes water spots, big time. I always washed my black cars in the evening or in the shade. Makes a world of difference.
You learn something new everyday here!