POSSIBLE CAUSES:
Non-galvanised iron nails have begun to rust, causing bleed-through to the top coat.
Non-galvanised iron nails have not been countersunk and filled over.
Galvanised nailheads have begun to rust after being damaged.
SOLUTION:
When painting new exterior surfaces where non-galvanised nails have been used, it is advisable to first countersink the nailheads, then seal them with a top quality primer.
Each nailhead area should be spot primed, then painted with a quality water-based coating. When repainting exteriors where nailhead rusting has occurred, sand the nailheads, then follow the same surface preparation procedures as for new construction.