Thanks for the feedback and advice.. I'll follow up all those suggestions.
Yes, they are SS bolts - my mistake - I think they've been exposed for so long they've developed a powdery coating which makes them look nylon.
Also noticed on closer inspection and digging that the water penetration seems to have come from a previous repair.. I think the frost we've had in the Med lately has found it's way into the small cracks in the repair and forced it open.. which would explain why it's only just become apparent.
So, to echo James's question - has anyone had any luck removing the genoa track bolts or is it really a drill out job? Maybe a good dose of WD40?
SV Haddock SM #189