I thought I'd tap the considerable expertise here.

From 2015 to 2019, Apple chemically coated their laptop screens with variants of some anti-glaze coating. But the screens have a 'best before' date. The coating sours, develops weird spots and loses it's charm.

My son is trying to sell his 2019 high spec Apple laptop. But it's not worth much if the screen is sucky. He's done the usual smart-assed-kid thing on google & youtube, but is no better off. Apparently there's variants, and what works on one is not a cure-all. He has ½dozen solvent/abrasive household things, but most of his sucky glaze is still there.

No wild ideas please, but if anyone has this problem sorted, I'd love a solution.

As it transpired, the considerable expertise here couldn't answer this. But my son found an answer.

He used "Armour Etch" and coated the offending layer for 2-3 minutes, then promptly removed it. The screen is as clean as a (very thoroughly wiped) baby's bottom.
It turns out fingerprints degrade the Apple screen coating over time.

This Armour Etch is for engraving glass. It is thoroughly nasty stuff, the principal ingredient being ammonium. Warnings are given about eye cover, or letting it touch you anywhere. Even farting whie the bottle is open risks a chemical explosion . But it works.