My thermaltake Volc 6+ had the same circular cutting lines in it s copper base and though fairly smooth I thought I would lap it just for sake of saving some time and trouble to have to remove it again and do it, After the first few moments across the sandpaper visual inspection shows the the center of the copper to to be e bit low and that was not good, But with a little more work it was done, flat and shiney.
This is not a knock on Thermaltake but maybe one that got past the inspections, It was a return at the store and aslong as the fan worked I could make the rest.
I also got a Thermaltake Chipset Blue Orb for my GFX card, Had to mod it fit ( nonstandard HSF on gfx card )< lapped it and it was very uneven, Good part out of it though is the lable claims 4600rpm fan and the MB reads 6500rpm.