This problem is characterised by rust-coloured, reddish-brown to black stains on the paint surface.
o Non-corrosion-resistant nails were used to attach siding, rather than galvanised zinc-plated or stainless-steel nails.
o Steel nails have come into contact with the air.
o Steel nails have popped up from below the surface.
o Excessive weathering or sanding has worn away galvanised coating on nail heads.
o Tannic acid from moist wood (e.g., oak) has reacted with steel nails, creating a black stain.
Repairs and Prevention
o If possible, replace steel nails with galvanised or stainless steel nails.
o If rusted nails can't be removed, then remove rust by sanding nail heads to bare metal and countersink them. Then, prime with a stain-blocking, rust-inhibiting primer. Caulk, fill, or patch depressed nail heads and sand smooth, and paint with high-quality paint.