I think you're too far gone to really fix it well. You could invest the time and clean it up, but as others have mentioned it would likely come back pretty quickly (unless you're really good at that stuff). Instead, I would just wash out out really good as often as you can and replace the hatch instead -- once it's dead.
Instead, maybe call a salvage yard and see if you can source the same color too... Check out car-part.com and source one local to you. I feel this site is one of the best kept secrets for sourcing parts... A quick search tells me that they're out there for $600-$1000 and you could get one with a spoiler, w/o a spoiler, with a backup cam, or w/o backup cam.
That'd be the way i'd approach it. Just inspect it good before you buy anything to make sure you don't run into the same issue.