What Causes It?
This problem occurs when non-galvanized iron nails that have not been countersunk and filled over begin to rust, and the rust bleeds through to the topcoat.
Sometimes this condition is caused by galvanized nailheads that have been sanded or have weathered excessively, and they then begin to rust.
How to Solve It
When painting new exterior construction where non-galvanized nails have been used, you should first countersink the nailheads, then caulk over them with a top-quality, water-based all-acrylic or a siliconized acrylic caulk. Each nailhead area should be spot primed, then painted with a quality latex coating.
When repainting exteriors where nailhead rusting has occurred, wash off the rust stains, sand the nailheads, then follow the same surface preparation procedures as for new construction.
Note: Images provided by The Rohm & Hass Paint Quality Institute.
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