I am trying to remove a rust stain with ascorbic acid

[Chuckiechan]

The plaster pool is dry, and I made a poultuce of ascorbic acid and covered it with plastic.
How long can it stay on a rust stain without causing damage?

 

[Texas Splash]

I don't know how ling you can leave AA on plaster, but if the stain is from iron deposited in the water, it should lift very quickly. If it's not, there may be something else to blame. Are you sure the rust (if that's what it is) isn't seeping though the plaster from underneath via the rebar? You might try placing a magnet on the area to see if it sticks - rebar.

 

[Chuckiechan]

It was a staple from a roofing job soaking all winter