Can't really comment on spraypainting or waterbase inks. Whatever you use they need to stick to the plastic, you don't want the ink to flake. I ususally print on blanks (no finish) but recently did a run on some discs that had a base coat of something.
We worried it wouldn't stick so tried a test, I use Dry Film General Purpose from ND graphics. This is a solvent based plastic ink.
It stuck in the test ( afinger smear) but finished up with a weird spotting when we printed, but looked like stars so the customer liked it. Best for you is to try what you have on the CDs you will get to print on.
this picture shows the spotted finish, and an indexing mark, printed with the green. (the little dot) you align this with a mark on the edge of the circle jig to print the black in register.
I tried waterbased speed ball, it looks kinda flat and if you rub on it it will kinda rub off. I tryed gloss acrylic but i forgot how that turned out. Better then waterbased but I dont think it was too great either.
I was under the impression that most CDs, the ones printed professionally, were printed with UV inks.
I could be mistaken though.
As usual, Richie is correct. The automated printers are like very sophisticated auto t-shirt carosels, with mini -UV dryers in between the print heads.
3600 per hour is standard, the cd moves on a suction platen under the print head, then the dryer, then next color, etc.