I think most cars end up with swirl marks, even if you use the 2 bucket method, there's no getting away from the fact that, at some point you will be dragging fine grit particles across the surface. That's why perfectionists use snow foam and power washers to loosen the dirt before touching the paint work.
Even my brand new car has a few swirls marks and I reckon it was my own fault. We got that Saharan rain with sand in it and I washed the car. Good rinse with the hose, used a good shampoo and a soft wash mitt. I'm not going to do that again. Just looked at my car now after a few sharp showers and it has actually washed most of the grit off. That's the best time to wash your car, not when we have had rain borne on southerly winds.