Electrolysis / Keel stepped mast
I have had same problem. The cure is to clean mast paint with two part epoxy paint, then keep the water away from the area. Instal a bilge pump that keeps the water to the absolute lowest point. Holes permitting water to leave the mast are also essential. By no means would I use Never Seas below water line or in wet areas(or grease). When submerged and wet this can actualy create a voltaic cell when sanwiched between two metal surfaces(even if similar metals). Also, if you use a wire wheel to clean aluminum, do not use a steal wire wheel. Small amounts of steel will always imbed into the aluminum and eventualy rust!!! Use a bronz pad or sand paper!