I've been messing with them for over a year trying to figure out a good way to clean them..
The ball swirls are basically small cracks... regular cleaning won't clean out the dirt that's in the cracks.
The Magic Eraser is made up of a special polymer that's microscopic, and able to get in smaller cracks then most cleaning tools... so basically, you can clean those small cracks out, it's like fine sandpaper though... so it'll remove all sheen, and if you go too much, you'll get rid of the ball swirl but you'll get rid of the paint too.
Once you wax it, the wax will get in the cracks, and now instead of black ball swirl marks, you'll have white ball swirl marks.
The best way to prevent this, is to clean the playfield with the magic eraser as much as possible, and get it as nice as you can... then clear coat the entire thing, then put the wax on the clear coat. The clear will get in the small cracks but you can't tell since it's clear! Very hard to do a great clear coat though.
Here's a picture of one I did a while back, you can see the white looking cracks in the paint, that's not ball swirl but it's the same principle; the dirt is out of the cracks, and now white wax is in them.