I owned a CS 27 for a long time. Most likely the keel is cast iron. Test with a magnet.
It looks to me like someone has 'botched a ( so-called ) repair over existing compsite
repairs.
All fibreglass boats will deteriorate over time. The place where the CS boats start to deteriorate is often aft the keel at the joining with the hull.
Tapping with a rubber hammer will tell you something about the hull condition( if it's thin) it will sound hollow.
You may have yellowing indicating the same.
There's really no substitute to taking it back to the level where you can see exactly waht is going on.
It's possible, referencing the
motor vehicle contribution here...that like 16 year olds with a first car...some damage/hole has been parthed over mesh? A patch that is now failing and rusting out the mesh.
Barrier coating of several layers of
epoxy is essential - when you have finally seen and sorted the ground down top layers. otherwise the same and worse will recur.
CS 27's of later years production like yours are very prone to osmotic blistering.
The earlier ones not so much.
You either have it or you don't seems to be the
rule of thin=mb and it varies boat to boat.
You should join the CS West forum in
Canada and put in the same question.
Wealth of talent there as well as here- but it's all CS specific.
As far as I know my CS and the others I know
All the above is good
advice about investigating, stripping down, taking off the existing heavy
paint layers and seeing what you have got