The tan discoloration looks very similar to the staining I get on the white scale in my pool. I can tell it is scale as the pool plaster is blue. My staining, mostly what appears to be iron, apparently comes from the sand/silt/mud/dust that blows in or carried in by dogs as we have no issues with our fill water.
I did the AA treatment last fall. It took longer than normal as the water was cool. The last stain to release was the tan staining on scale. I was anxious to let the doggies swim again, after two weeks of treatment, (many chem reactions slow down as the water gets colder) so I didn't go any longer with the aggressive AA treatment. There was still some slight staining on the scale on bottom of shallow end. I slowly brought the pH up to 7.2 and slowly brought up chlorine. I kept generous amounts of sequestrate in the pool all winter with pH hovering around 7.2 (but not below). The tan staining continued to release all winter as did the scale. I can see a hint of tan in the shallow end but I'm not doing anything other than keeping generous amounts of sequestrate and every other day or so brushing. Keeping pH right at 7.5.
I agree with others about NOT doing an acid wash. I lucked out with my contractor when I had the pool replastered in '96, which apparently was a great job, as my mistreatment of the pool prior to finding TFP, last year, and three aggressive acid washes I did didn't destroy the finish. A friend and I did a very drastic acid wash, summer of 2008, and I can see some areas that were roughened up more than from the other two acid washes. So no more acid washes for this pool.
It's pretty amazing what an AA treatment can do if your stains are certain metals and AA is far less destructive.
BTW... I don't think anyone mentioned but get the areas of plaster you are going to test well saturated with water before testing. Possibly hold a wet sponge on spots if you don't want much water going into pool.
gg=alice
You made the right decision!
