well i ripped off the rusty beaten up quarters on the road car, and am going to replace em with the new ones i have, they are from nispares and are coated in a black paint

to paint them what do i do?

do i just sand back lightly the black and paint straight over, do i need primer on top of it, or do i go back to bare metal???

I'd sand them back with wet and dry, lightly prime them, and then paint them properly.

You can paint over the top though. But I'd at least wet and dry them and make sure they are VERY clean before applying colour. This is assuming theres no bare metal showing through.

some of the new panels on my ute came covered in that black paint- basicly i gave them a light sand and then primed them again very lightly to give the paint a better sticking surface. Then you just spray em- works a treat.

make sure that the panels are ultra clean before you spray- no dust

Before you spray but after you have repaired & prepared & cleaned wash the panel with a clean rag doused in wax & grease remover then dry with another clean dry rag. I usually do this in 300-600mm areas so you are wipeing it dry not allowing it to dry. Also have the panel set up ready to paint so once you have done this you don't touch the surfaces before you spray. You have to be a real old woman about cleanliness when painting.
!!! Don't forget to wear a mask.