In Antwort auf
Saw the same problem at my keel on BA24 of 1976.
After removing several layers of anti-fouling, the rust was showing at the surface of the lead part of the keel only.
Rusty stains could easily be removed from the lead by a knife.
Suggested root cause: Some iron from inside the negative mould/form of the keel was sitting just at the surface and eventually got in contact with salt water.
1. Removal of anti fouling above (3cm) and below (5cm) the seam between hull and lead keel until the lead is fully shining.
2. Removal of all rust stains by sharp tool.
3. Remodelling the surface with epoxy and several layers of paint/anti fouling. At this step the epoxy covered both sides of the seam. After some hardening time i cut an open line (4mm blade) along the underlying seam and filled it by Pantera or Sikaflex.
4. A new layer of anti fouling finished the job.
After first season in water, no new stains observed.