I'd mask off the part you don't want spraied, on the INSIDE of the plex, then hit it with a coat or two of good quality black. I don't think you'll want to sand, since the sand marks will show through, and you definitely don't want to prime.
Now, if you do decide to paint the OUTSIDE of the plex, it's a whole different story. In that case, i'd sand AND prime, but I suspect you'll get much better results painting the inside.