Depending on what you have sanded it with, will lead you to the right compound. A Harbor Freight buffer will be fine, but i might suggest that you get a 3M Perfect It III compound foam pad, and Perfect It III rubbing and Polishing compound. THe Foam pads are much easier to use, and will be harder to burn through (worth the $ IMO)
Sanding with less than 1200 grit will be more difficult to polish than a 2000 grit final sand. More work in the sanding department, but less polishing, and a better result. Do a little at a time, and with the Perfect it, you dont need much compound to do the job. make sure to be careful around the edges, and lift the trunk and hatch, and open the doors while buffing the edges. Doing this while the pad is spinning away from the edge so that it doesnt catch it, will lessen the chance of burn through.
If you already know this sorry for the info, but learning the hard way isnt fun. Good luck