Even sealed up night vision equipment somehow seems to get crap inside it. Yes if you open your unit it will have to be reurged. The cleaning part is alos somewhat dificult as you can scratch the back of the "screen" and then you now have a blemish that you will ALWAYS see.
SIERRA II ALPHA
I gave up trying to get my PVS7 completely clean... Every time the lense housing is unscrewed/screwed from the body, it makes tiny plastic shavings that get on the lens...
I purge them syself with a plastic bag and nitrogen we use at work to fill gas charged springs.