Are you refrigerating your cakes? It is very hot and humid here in Arkansas and the ONLY time I've had a problem like this is the ONE time I refrigerated my cake.
Humidity isn't a problem with a cake that is left at room temperature. I've never had a problem with "sweating" fondant if the cake is never refrigerated. I never cover/wrap my fondant cakes with plastic wrap either .
I don't use perishable fillings so there is never a reason to. I do use milk (half and half) in my buttercream but it also requires no refrigeration.
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