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WM is correct, and the main reason it forms in the first place is that the tanks are not kept full (and were not while in military service). Mechanical action is needed to break it free. For the screen it needs to be a very gentle action to break the edge of the crud, then - TRY in a small spot - some 'Gumout brand' spray carburetor cleaner. it's almost as powerful as brake cleaner. I've cleaned mine up that way, and since the tanks stay full between uses, it has not re-formed. 