My experience with damp rid and the plug in regenerative ones is they are useless. I'm sure damp rid actually draws moisture into a safe. I was changing mine on a regular basis in a safe that was not in a garage but in a very dry room in the house. The only issue I was having though was a mild forming of white mould on leather slings.
I went with a cupboard heater and seperate thermostat and it's all good now. The little tube ones sold by gun shops are under nourished though.
There may also be a risk of overdoing it and drying out seals and lube in scopes, suppossedly.
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