Used it on my Westsail to erase the chain plate bleed on the hull. Brushed it on and rinsed after a few minutes, stains were gone with minimal labor. Bought a 25# bag of it from a chemical supply place for not a lot of money and it lasted me for decades of bleaching wood and removing rust stains. Use Barkeepers Friend for keeping stainless bright and shiny. It's a mild abrasive with Oxalic acid.

Phosphoric acid is the active ingredient in Ospho metal treatment. It's good at converting rust to an inert black ferrous oxide in preparation for painting. Didn't seem to do much for removing rust stains, however.