At the time of the discussion about restoring headlights, I needed to do one of mine. The car is a 2002 Renault, and in 2011, a pheasant took out the passenger side front lamp unit. I replaced it with a used but very clean unit, and it appeared much brighter than the existing driver's side one. Over the years since, the driver's side 'glass' got murkier and yellower, while the passenger's side remained clear. The perceived brightness on the road was clearly suffering. I bought a Meguiars restoration kit, that requires no drill, and attacked the driver's side. It got to the point where there was no change to be made to the surface, even though it was still nowhere near as clear as the passenger side.
But application of the sprayed sealer was a revelation, suddenly it is as though the headlamp unit is a new fitting. Now the passenger side is the murky side, and although it didn't need restoring before, it does now.
Usual disclaimer re: Meguiars, ie no relationship.