The problem is, what you're polishing (the first time) isn't the lens material, the actual polycarbonate. It's the hard clearcoat that protects the polycarbonate from abrasion and UV-induced degradation. You sand that off and yeah, you can make the polycarbonate look nice...briefly. But now it's exposed, with no hard coat, and that is a death sentence for it. And no, "Plast-X" or any other such product isn't going to help.
More permanent results require a real, serious, UV-blocking, abrasion-resistant hard coat. The only kit with a hard coat worth messing with (i.e. long enough protection to be worth the effort) is this one, or good results can be obtained with this method.
Keep in mind, though, that the lens is only one part of the headlamp. Reflectors degrade with age, too, and there isn't a kit for that.
More permanent results require a real, serious, UV-blocking, abrasion-resistant hard coat. The only kit with a hard coat worth messing with (i.e. long enough protection to be worth the effort) is this one, or good results can be obtained with this method.
Keep in mind, though, that the lens is only one part of the headlamp. Reflectors degrade with age, too, and there isn't a kit for that.