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So the easiest way I have found to clean my thermos and cycling water bottles is just to get Denture Cleaning tablets and dump them into the container then fill with boiling water. Let sit overnight then repeat. When I was in the army I used this method for my canteens which were a plastic. Sometimes it takes 2 or 3 times but has never failed me yet. For larger containers give it 2 tablets. Even with the canteen I lost for 3 months with who knows what inside it. You do need to repeatedly rinse the thing afterwards but it works.
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