Thanks Marie,

I need a fair amount since I have to clean a 1000 gallon gunite 'kiddie' pool badly stained with iron.

I would guess the best approach is to drain, partially fill with FC level at 0, get ascorbic acid concentration high and circulate until stains are removed.

Is rubbing with wire brush helpful? Do I need to buffer pH? Is there any risk to the plaster if pH is low for an hour or two?

Thx