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Take it to a sony service and let them clean the device.
Yes,in this case they might replace the unit.
Then i dont understand how could any dust go under the lens. Can you post a picture or something of the lens. If the device is waterproof and there is dust under the lens, am i stupid to thing you putted the device into sand with opened covers? There is no way of any dust coming into the lens if the device is waterproof and all covers are closed.