I've tried everything over the last six months, air dusters/blowers, lens cloths, pre-moistened lens wipes, lens brushes, even a
Speck grabber!
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All of these have just resulted in moving the small particles around within the Rift without actually removing them, with the strength of the lens even the smallest particle can be quite visible.
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The only thing I found worked really well was a mini attachment kit for a normal vacuum cleaner, to suck the bits out! (obviously with great care so as not to scratch the screen)
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Universal Mini Micro Vacuum Cleaner Attachment Tool Kit
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I now keep my Rift in plastic bag whenever it's not on my face, just to stop dust collecting on it (food freezer bags work well).
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I also recommend these machine washable covers for the Rift to help keep it clean when it is in use:
VR Covers