This item sold by TheCoolCube had an oil film on the inside of the digitizer when you removed the protective film. It should be clean and free of dust, fingerprints, etc. You don't need to clean the inside of the digitizer before installation. I've installed a lot of screens so I check them out, but for those that don't, you'll need to remove, clean, and replace this part. The problem with needing cleaning is that it allows dust, lint, and other debris to get between the LCD and the digitizer. Canned air doesn't work well when trying to collect 99% lint free material as it will kick up dust. High-quality microfiber towels fix this, but are a bit pricey. Who needs a speck of dust hitting the screen.
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