There will always be moisture condensing on the fuel cap due to temperature swings from day to night. You can lessen the amount by keeping the tank full. The issue is with the lousy plating on the cap, which has worn off. You can stop it from further rusting by smearing a light film of wheel bearing grease on it every so often.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked