Removing a Rust Stain in Porcelain Tub?

I have a white porcelain bathtub that was heavily stained with rust when rusty drain clearing cable was left spread in tub. Sprayed the tub, sprinkled Bar Keepers Friend on stain. Wet cloth and lightly scrubbed the tub. The rust disappeared 90% first application. Repeated day latter only traces of faint rust shows I am not sure how many times I can use BKF, their web site says remove after one minute.

By UncleDaveM