So I picked up this 94 GXE grill from ebay for $35 with emblem and I am in the process of painting it black. Here is my problem, I sanded a lot of the chrome off and when I was sanding the primer coat lightly, the bottom part of the grill, the primer starts flaking off. Do I need to sand ALL of the chrome off? That coating is so thick, it seems to never come to the bare plastic when sanding.

You need to be sure that you get as much chrome off as possible before priming/painting. Also try to find a paint formulated for plastics.