Now you did it, Mother Nature monitors this forum and she will take this a a personal afront, and brew up a week of 20 degree weather with 12" of snow !!
For summerizing: According to Jerry at LeisureLine, wash everything down with baking soda disolved in water, this neutralizes the acid in the coal/ash.
I recommended a rust/corrosion proofing product called LPS-3. It is used in the aviation industry as well as others to seal up parts for storage or use in corrosive environments.
The LPS-3 leaves a waxy layer that will greatly reduce oxygen getting to the underlying surface.
WD-40 or similar thin sprays leave behind only a thin layer of silicone, which does very little to prevent corrosion.
I would wirebrush, sand and paint the inside of your coal hopper, this is better than any other method to stop rust, and preserve the parts.
Take care, just don't summer-ize the snowblower and snow shovels yet, maybe MotherNature will be appeased