Quote Originally Posted by Sumo1 View Post
I've just started reading this thread and don't mean to get in the way but there may be others wondering like me--what does a sequestering agent do?
A sequestering agent 'deactivates' metal ions in the water so they don't cause staining or, in the case of calcium (which is also a metal), scaling.
My gunite pool has has an old plaster coat with several stains but I never really thought I could remove them so I just dream of a new plaster job.
It is possible to remvove certain stains from plaster but sometimes the process can take a while.

This thread has some more information about sequesterant and chelating agents and reducidng agents.