The crusty lookin' areas were simply areas that had not been sanded enough to remove the factory coating.
If you have crust, it is the factory coating degrading and you need to remove that coating, generally in its entirety.
Sanding can be a tedious process, to be sure, but the results are impressive when done right. The 3M kit seems to excel in this area.
Any time you "restore" a headlight, you will have to maintain it with a polish, and the Mothers PowerBall 4Lights kit includes 8 ounces of polish and there's enough life in the polishing ball to maintain several cars for some time to come. There's even an Xpel coupon if you opt for a protective film approach for future protection from impact and UV.