Have a new steel/enamel bath that, due to lack of space, has been stored
outdoors awaiting installation, rainwater has pooled in it, with
decomposed leaves etc, so not ideal. I pressure washed the green gunge
off it today but a tidemark stubbornly remained in the bottom, even the
p/w turbo nozzle wouldn't touch it.

Tried a product called 'Cramer Email Star' applied with a damp
microfibre cloth, doesn't need much more than gentle rubbing and then
rinsing off, spotless ... recommended!