Assuming all the original coating was removed when you restored, when you see heavier deterioration at the top, it's probably be due to that surface receiving more UV than the rest of the headlight. If you wait long enough, the entire headlight will cloud starting from the top and working down.
We use a polyurethane coating, not minwax/MS but probably similar. However, our coating is much thicker than a 50/50 mix. The thicker the coating, the longer the lifetime, but the more difficult it is to apply.
We now only use poly for low end restores (fleets, buy here-pay here, etc.) but we are guaranteeing for 1 year. Few callbacks, but some, especially PT Cruisers because of their horizontal surfaces. They get hammered with UV.
You might try the following to get better lifetime:
Don't sand any finer than 1500 grit. The rougher the surface, the more surface area to stick to.
Make sure surface is spotless before applying poly. We scrub twice with blue lint free scott towel and Propanol. Don't use any alcohol with water in it like rubbing alcohol.
Make sure the cup you mix in isn't coated with wax, like a paper cup from McSonald's. HDPE or PET is best.
You want as heavy a coat as you can apply, Not familiar with Minwax, but would start ay 100% if possible. A heavier coat is thicker and dries slower, giving you time to work the coating. We apply with closed foam applicators wiping on with horizontal strokes, followed by wiping vertically. This smooths out the coating and gives you a uniform thickness. This should last longer. If you're charging money for your restores, I would try using a better coating like 2K LPE. This is what we use on most restores and warrant for 2 years (with 2 coats). The actual cost per headlight (2 coats) is less than a dollar. Poly is probably a quarter, Not worth the risk.
ray6
