I have a pool plaster problem. There are greyish/brownish mottled areas--your posting that talked about "hydration" problems sound similar but I'm not sure . I've uploaded a picture and hope for some opinions on what my problem is. I have 20,000 gal chlorine pool with pebble tec that is 10 months old. I noticed something in some areas of the plaster soon after it was filled and was told it was just areas of more concentrated aggregate. Over time the problem has worsened but is only in a few areas all along one side of the pool----in the curve below the bottom step in the shallow end, on the sun shelf and the face below it, the wall/face below the spa water fall, and on the spa seat. The plaster company rep just came out and took a water sample that he said would be tested for iron to see if that was causing staining. He checked my ph and said it was in range---said chlorine was high and that high chlorine levels can cause staining. I know well enough that calcium and ph levels could cause plaster problems over long periods but I've never heard high chlorine levels are a problem. He said they would do a light acid wash to see if that gets rid of the stains and that we need to determine what is causing the stains to begin with--thus the water testing. He did say that if that didn't work, they would replaster. Looking forward to reading what you think. Thanks!