fiberglass, slight yellow whole pool stain/coating

[waterbear]

You will need about 1 and 1/2 quarts of the purple stuff (or any other HEDP based sequesteant that uses 1 qt per 10 k gallons) initially. You might need to add a maintenance dose weekly, every two weeks or you might be able to get by with a monthly maintenance dose. I would start doing it weekly for a few weeks and then slack off to every two weeks. If the stains start to reappear go back to weekly. A lot depends on your chlorine levels, which can break down the sequstering ablility of HEDP. If you are staying stain free on a two week schedule then try only adding the maintenance dose monthly.

For a lower priced alternative I have had good results with Proteam's metal magic but even a ''house brand" will be equally effective. You want the active ingredient to be HEDP or phosphonic acid or a phosphonic acid derivative. If the ingredients are not listed on the label consult the MSDS for the product.

As far as testing sequesterant level in the water, Jack's Magic has a test kit for it (very expensive, IMHO) but it really is not necessary in most cases. LaMotte also has a sequestering agent test kit available for pools. Taylor has test kits but they are made for the boiler/cooling market and not for pools and are extremely expensive.

If you just observe your pool and learn how often you need to add a maintenance dose to keep your pool stain free you don't need to test.
My pool only needs a HEDP based sequesterant monthly but if I use an EDTA based sequesterant such as MetalFree I need to add it weekly or every two weeks.

Edit: If the staining is not extensive you can often eliminate it with a double or even triple dose of sequesterant and by dropping the pH as stated earlier in the thread. Put the sequesterant in first and test the pH since the sequesterant will drop the pH some on its own. This is usually more expensive than treating with ascorbic acid but it does not cause the chlorine demand problem. It just eats up the TA!

[Sumo1]

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? 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.

[waterbear]

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.