Islandswan, I have the same problem (I think) and my plaster installer offered to do a no-drain acid wash followed by a drain and re-fill for $500. I decided it wasn't worth it. My plaster, while not perfect, doesn't look bad in full sun. Only at low light levels do you really notice the mottling. I've learned to live with it and I hope the mottling is only a cosmetic issue. My pool is coming up on two years old and the plaster is showing no signs of wear other than the mottling.