I don't participate in these threads because my experiences and processes don't match the commonly given advice. But one thing I don't see talked about a lot is to make sure that the OEM coating is completely removed. Usually on a degraded headlamp the coating will be gone on the upper areas but still may be present in the less faded areas. It needs to come off because it's solvent resistant and will not be as transparent as the rest of the lamps when completed. This usually must be done with a fairly coarse grit like 320.
Also, I have the best luck with slow clears. Fast materials that are high in acetone can result in a less transparent, somewhat milky result. Most people don't notice, but fast clears and reducers are best avoided if possible. Some plastic blends seem more susceptible to this than others.