Reddish-brown stains and spots on the paint surface.
Non-galvanized iron nails have begun to rust, causing bleed-through to the top coat.
Non-galvanized iron nails have not been countersunk and filled over.
Galvanized nail-heads have begun to rust after sanding or excessive weathering.
When painting new exterior construction where non-galvanized nails have been used, it is advisable to first countersink the nail-heads, then caulk them with a top quality, water-based all-acrylic or siliconized acrylic caulk. Each nail-head area should be spot primed, then painted with a quality latex coating. When repainting exteriors where nail-head rusting has occurred, wash off rust stains, sand the nail-heads, then follow the same surface preparation procedures as for new construction.