The black coating is very fine and comes off after a couple times you screw them in.
They are also very easy to dent so if you are looking for a clean looking rack maybe you should be very careful or buy something else.
I said clean looking rack because they are slotted, which is very uncomfortable to drive in and the only reason to buy them is because you want that look :D
Besides, if you just need them to tighten up some rack modules they work just fine and will probably never fail.