Reddish-brown stains on the paint surface are caused by nail head rusting.
Nail head rusting can be caused by any of the following:
Non-galvanised iron nails that have begun to rust, causing bleed-through to the top coat
Non-galvanised iron nails that have not been countersunk and filled over
Galvanised nail heads that were damaged during installation or have rusted after sanding or excessive weathering
If you’re painting a new exterior construction where non-galvanised nails have been used, countersink the nail heads, and then caulk them with a top quality, water-based all-acrylic caulk. Each nail head area should be spot primed with zinc rich primer such as , then paint with a quality latex coating.
When repainting exteriors where nail head rusting has occurred, wash off all the rust stains then sand the nail heads and follow the same surface prep procedures as you would for new constructions.
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