Reddish-brown stains 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 nailheads 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 nails then caulk them with a top quality, water-based all-acrylic or siliconized acrylic caulk. Each nailhead area should be spot primed, then painted with a high quality latex coating. When repainting exteriors where nailhead rusting has occurred, wash off rust stains, sand the nailheads, then follow the same surface preparation procedures as for new construction.
Source: Paint Quality Institute