if its your first time doing CGs you will mess it up. theres a whole art and science to polishing as well, 1500 grit alone and some cape cods wont do it for modding jobs. depends how deep any scratches are, pitting, quality of the metal etc. i start at 320 grit then 400, 600, 800, 1000, 1200, 1500, 2000, 2500, 3000, 4000, 6000, then green polishing compounds and a felt pad, depending on how its looking i may using a diamond paste as well, also the brand and intended application of the paper also makes a difference. if you just want to brighten up the already factory polished case then a cape cod will do fine.
i say either practice on a disposable case or outsource it if you really want great results.