I decided to use Jack's Magic "The Copper and Scaling Stuff" as I had not done an acid wash in over 10 years, and despite doing the iron treatment and adding enough of "The Purple Stuff" faint stains as reported by others still returned, despite sufficient ppm of "The Purple Stuff" sequestrant.
I believe that these stains are due to scaling. My un-professional opinion is that the AA treatment removes surface staining of the iron which does make the pool look good for about 2 or 3 weeks, but then keeping the pool CSI slightly negative at -0.20 slowly allows the more ingrained iron to show through. I verified this by marking out a particularly noticeable stain on my gunite steps with large stone pebbles and sure enough it returned slowly with the same shape.
I therefore decided to do this treatment, at their required specs, which they informed me today will take from 1 to 3 weeks with twice daily wire brushing, 48 hours non-stop circulation, then 10 hours a day circulation after that. The balance was put within their optimum requested setting. my actual settings prior to adding their solution last week was pH 7.2, Alk 80, CH 350, CYA 28, Salt 3200. Then I added per their instructions, 1/2 gallon of muriatic acid, 10 lbs of "The Copper and Scaling Stuff", and 32 oz of the "The Purple Stuff", and then brushed it immediately over the pool surface with a wire brush. I have been brushing the pool twice daily and stains are getting fainter, I have been maintaining the FC at around 2.5, the pool uses virtually no chlorine. During the first 48 hours non-stop circulation I turned my Pool Pilot SC-60 to Power Level 1, at 1%, yes just 1% percent and it maintained the FC at 2.5. The CSI index is a negative -2.16!
No damage to walls or equipment is occurring the CH has risen from 350 to 425, the pH is 6.2, and the Alk is 20. All of which they said is good. I added an additional 32 oz of "The Purple Stuff" after 48 hours, which they said was a good idea. Once the stains are gone they stated to add baking soda only to up the Alk to 80, but no more than 3 pounds of bicarbonate per day, per 10,000 gallons of water, until it settles. And to backwash weekly if the pressure rises.
I hope that once done this will remove all scaling and stains. They stated that after 3 weeks, they originally stated 10 weeks but on the phone today said 3 weeks, if the stains are not all gone, we should use their test kit to see what they are. In sunlight one can see no stains whatsoever but when the pool falls into shadow the faint stains are visible.
If successful, they suggested keeping the pH at 7.20, Alk at 80, CH at below 400. Assuming these numbers I would get a CSI of -0.44 at 80F, is this OK on an ongoing basis, the salt would be 3200 and the CYA 30?
Most of the iron comes through airborne dust and I think virtually none through our water system. I will be buying an iron test kit.
I really would like to solve the ongoing shadow stains returning that is why if you think it is OK I would be willing to keep my CSI at -0.44. The pool measures 0 ppm for copper.
Update: Just received the Jack's "Sequest Test Kit" in the mail. The sequestrant "The Purple Stuff" measured only 10 ppm, a maintenance dose for iron but not the 18-20 ppm required for in-pool acid washing. This after adding 32 oz was initially added and then again 32 oz 48 hours later. So it is using a lot, added another 15 oz, actually 18 oz giving 3 oz reserve. I think the "Sequest Test Kit" is a must if you are going to do this and might want to use it daily to speed the process along. Their tech support is very quick to connect to, they are located in Florida and are, I believe, open till 5pm Monday thru Friday; they are very helpful and pleasant.