The best water paste is "Kolor Kut", which changes from orange to bright red in the presence of water.
Kolor Kut have also introduced a new version which is excellent for Mogas use - will also detect water contamination and phase separation in Mogas containing Ethanol (not that we should fly with the stuff in theory
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Google Kolor Kut for stockists. (Dont buy too much coz 1 tube should last longer than most aircraft if used properly)