I tried plumbing tape but still had leakage around the cap. The iodine vapors were starting to corrode some of the metal in my PSK. I found that 2 layers of electrical tape around the outside overlapping the cap and the bottle, prevented any vapor leakage. The tape should be replaced once a year.
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"The reasonable man conforms himself to the world around him. The unreasonable man conforms the world around him to himself. Therefore, all progress is dependent upon the unreasonable man." Unknown