Any double that gets properly wet when hunting in the rain needs to be stripped down ASAP IMO, dried out, and all internal parts recoated with whatever rust preventative/lube you use.

Water enters the action quite easily at the safety catch slide, and where the triggers pass through the trigger plate.
The fore-end is also an area for concern especially with ejector guns.

Rust can form on lock parts and because they are hidden from view you may never know untill it is too late unless the gun is serviced after being used in rain.

This servicing is beyond the ability of many owners and in this case the gun needs to be given to a reputable doubles gunsmith for the job.