Painting my Car: How much should I sand?

Yesterday I fired up my DA and started on my car to begin preparation for painting. To my surprise, the paint seemed to disintegrate and turn to dust and did not clog the paper as I am accustomed to. The previous owner apparently used black primer rather than actual paint perhaps? At any rate, the surface is rough and makes it pretty easy to see where I have sanded. My question is, should I go to metal, the old paint or till it’s smooth. I plan on going from 180 to 240 with the DA, then wet sand with 400 before I shoot paint. Just wondering how deep to go now.