Rust

Reddish, brown (or sometimes black) stains appearing as blotches or streaks on surface or paint. See also Tannin Staining.

Steel in contact with air and moisture will rust (e.g., nails, steel railings, damaged galvanized, flashing, etc.).
Steel nails may develop black discoloration when in contact with damp wood that possesses a high amount of tannic acid.

Prime bare steel as soon as possible with a rust-inhibiting primer or finish.
Replace steel nails with galvanized or other non-rusting nails.
Rusted nails that cannot be removed must be sanded to shiny metal; countersunk; coated with a rust-preventive primer; and then caulked, filled, or patched; and topcoated.