I have an in-ground pool, with flagstone coping around the edge and a nearby bench style retaining wall. We live in Texas. The flagstone was called "Mexican Gray." We've had the pool for two years now. After heavy rain there is substantial rust discoloration that shows up on the flagstone and on the mortar in between the flagstone. It is most pronounced on the vertical face of the retaining wall. The pool builder has come out twice to "bleach" the stone (that's what they called it). They basically painted a white substance over the affected areas, let it sit for two days and then came back to remove it. The process did temporarily remove the stains, but now after more rain, the same rust colored discoloration is back. The pool builder said that this happens sometimes with this stone (although they didn't mention it when we selected it) and said that there must be some iron in the stone which would eventually all leach out over time and the problem would disappear. This doesn't really make sense, especially since the problem appears to show up primarly on vertical walls and not on the horizontal flagstone coping around the pool. Does anyone have any experience with this problem? Are there any other solutions? Also, is there any chance that the "bleach" substance they keep painting on my stone will weaken it or cause damage to the stone? The rust has gotten bad again and I am getting ready to call them out to perform the same bleaching process again, but am concerned it might be damaging my stone in the long run. Thank you for any help or suggestions.