We have a cast iron ballast here. A slab on a grp stub. SS bolts, paired, 10 pairs. Bolts are in pockets and can be retrieved and controlled from within the boat.
The procedure (where ours was built) was to dip galvanize after casting. Later it was epoxied (no glass) over. Once the galvanizing gets scratched, water
can get in and the thing starts to rust. No problems in boats hauled our annually.
I have seen ballasts like this in pristine condition as well as completely rusty.
If you find a rusty one, sandblast to bare metal, epoxy
. Depending on how much epoxy
you use and how well you do the job, you are fine for 3 to 10 years or thereabout.
If you have a very seriously rusty cast iron keel you want to make sure the bolts are fine. If they are not, renew them.
Do do the job very very well on a very very rusty one it may require to drop the keel. You normally unstep the mast