800 grit finish for flat finishes. 1000 grit minimum for gloss. 1200-1500 for metallic.
For acrylic paints, the major consideration is to get the plastic surface CLEAN. Wash it in Windex or a Simple Green solution, rinse thoroughly in tap water, followed by distilled water. Wipe dry with a lint free cloth (I use Swiffer pads). After that, never touch it with bare fingers.
Acrylics dry quickly, but achieve full cure more slowly than enamels. Wait at least 24 hours before masking over acrylics.