I'm pretty sure your issue can best be described as an "oleophobic coating" issue. The stains aren't actually stains but areas of display where the oleophobic coating has been worn off or was never applied correctly(during manufacturing):
A quick way to test if the oleophobic coating has been worn down is to drip a few drops of water over the different areas of the display.
The water will form spherical drops (beading) where the coating is intact but lie more flat (sheeting) in the areas where there's no or little coating.
On a smartphone display (left is with coating, right is without):
How it looks on a big piece of glass:
If it is indeed just the oleophobic coating that's worn off then you can reapply it yourself fairly easily:
A quick way to test if the oleophobic coating has been worn down is to drip a few drops of water over the different areas of the display.
The water will form spherical drops (beading) where the coating is intact but lie more flat (sheeting) in the areas where there's no or little coating.
On a smartphone display (left is with coating, right is without):
How it looks on a big piece of glass:
If it is indeed just the oleophobic coating that's worn off then you can reapply it yourself fairly easily:
Oleophobic coating – what it is, how to clean your phone, what to do if the coating wears off
By this day, you've probably seen the term "oleophobic coating" on some tech-selling sites – it's sometimes mentioned in phone specs, while at other times – screen protector manufacturers will boast about it on their product's page. But what exactly is this strangely-named substance, what does...
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