And if that's the case, and it is a crimped/sealed gauge, you will need to replace it. What it sounds like, is that it gets warm when you use it, and the moisture evaporates, then when you shut down, the warm gauge, as it cools, draws in moist outside air, so when you start it, the heat then makes the trapped moisture slowly evaporate? Does that sound right?
The only other possibility is that it IS sealed well, but you have trapped moisture, that you need to purge. Have you tried removing a rubber seal in the back, driving the car or parking it out in the heat, then replacing the rubber seal? (if there is a seal)
??
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