Well, in terms of optical clarity, 600 grit isn't even close. It should not be too hard to find 1000 and 2000 grit papers (although cloths are better because you'll want to use them wet) locally. Model car builders like me use abrasive cloths that start at 2400 grit and go in about six steps (3000, 3200, 3600, 4000, 6000, 8000) up to 12,000(!) grit, which leaves a reflective shine. Still, a little work with talc and a cloth will improve on that. A nice coat of wax finishes it off. If you can't find these grits, try searching for the Novus® plastic polishing system. The general principle is, use whatever coarse grit will remove the visible defect and then use the progressively finer grits (don't jump from 600 to 4000, for instance; it will take you all day) to remove the marks from the previous sanding. Be patient and your results should be great.
