Do both a fan and a dehumidifier.
A buddy of mine is in the Limousine business. He just bought a house from a guy that was a contractor.
Hillside ranch, with a walk out basement. One of the selling points was the attached garages. (Yes, plural.) One above the other.
Up stairs was a pretty typical 3 car setup, except one bay is 30 feet deep. But down the driveway on the hill to the back of the house was the “basement garage”. It is 20 feet wide, 15 feet high, and 40 feet deep with a loft over half of it. It is under the 30 foot bay above.
He figured he could park at least 2 limos down there and one upstairs. It would save him a lot of storage fees.
But the back end of the downstairs one was nasty. Dark and damp. The contractor would park his dump truck with the bobcat on a trailer down there.
After we installed lights, I hung a window fan from the ceiling under the loft, blowing at the back wall. I put a dehumidifier in the back corner with the drain hooked up to a garden hose running along the wall all the way to the door 40 feet away. I used a 50 foot hose and just cut off the last 10 feet. The door comes down on top of it.
Both the fan and the dehumidifier are never turned off. There is always a damp spot at the end of the hose coming from the dehumidifier.
It has done wonders. In the 2 months he has been there, that black hole has become very nice.
This solved the rusting problem, but I had to mount the dehumidifier up on a shelf to reduce the clogging from dust, etc. I tried adding a furnace filter but it reduced the airflow and the motor overheated.