Practically any spray will void the warranty - there are quite a few, from matte spray paint, matte Fixative (used by artists to seal pencil and charcoal drawings) and photographer's matte spray (used to take the glare off a subject being photographed for display). Oddly, hairspray will do the same thing. Problem is, all types will probably etch and melt the screen slightly, and will probably shorten its life. There are also matte overlays as well, but they tend to darken the screen.
There is a reason for the glossy (or Brightview) screens - and that is they tend to be able to be seen in almost any environment. Yes, they also can double as a mirror in some cases, but to dull them down will ruin their advantage over the old flat matte screens.
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