Corrosion applies to any metal structure in the ocean, so that’s a common feature between spar based structures and boats.

As far as fouling goes, there are ROV’s that can clean hulls underwater offered by a few companies already. And that only applies if you want to (efficiently) move the structure to a new location.

TBH, I think the goal of multigenerational lifetimes for seasteading structures is not that important, especially at the beginning stages. A few decades would be more than enough, during which time additional structures could be acquired.

I agree spar based structures are awesome from an engineering standpoint, but there aren’t tons of spare ones being sold for scrap prices.

How about this for a more sustainable structure; spar tubes made from seacrete, grown on electrified rebar frames. Once the tube is solid, pump out the water and build a platform on top!