The string bit was usually done with "Summertime." If he had a bass player, he'd sing with the bass and tune the string up at the end.

He told me the fingernail problem was actually a kind of psoriasis (white man's disease, he'd say) and that he periodically had the nails removed surgically.

I saw him first in 1959 performing with Odetta at Champaign, Illinois. I was never the same, He was THE blues performer.