Nailhead rusting: Rust spots come through the paint. Preventable with proper surface preparation and priming.
Rust stains probable causes: Improper surface preparation. Use of nails not protected against rusting. Failure to countersink and apply rust inhibitive primer. Painting over rusty nailheads, in a short time the rust will bleed through again.
Nailhead rusting solutions: Countersink the nails, spot prime and use acrylic caulk. Rust spots will reappear if you use other rusting metals without prescribed preventative measures. Specify galvanized, aluminum or stainless steel nails.
Primers & exterior finishes: Priming and proper surface preparation are essential with a corrosion resistant primer. Select a top quality exterior paint in the color and sheen of your choice.