My pool is a 16x32 inground plastered gunite pool built in 1965. It has been painted many times, the most recently 5 years ago using Olympic Zeron epoxy paint. I've been very happy with the paint. I had previously painted with chlorinated rubber paint so I had to use a special intermediate primer, but the Zeron has looked good and held up well. This spring, there were some chunks of plaster and cement that fell out of the wall that needed to be repaired. I drained the pool and have been cleaning the finish. I will then patch the damaged concrete, paint the patched areas with paint, then refill.
In cleaning, I have found some stubborn stains in the deep end of the pool. They seem to cover the entire bottom bowl of the pool following a low water line. The stains are not responding to pressure washing, acid washing or TSP. I was diluting the muriatic acid, but this morning I tried spraying it on full strength using a 30% acid. I cannot see that it dented the stains at all. The stains are a dull brown color with splotches of white (which is the color of the epoxy paint). I did not do a good job of covering the pool last winter. I didn't get all the water bags on and it fell in so I just left it and pulled it out in the spring. There were a lot of leaves. Perhaps tannin stains?
I set a triclor tab on the surface for half an hour and it turned it yellow. I am thinking I may just continue with the rest of the repair, fill the pool, and hope the chlorine fades the stains over time. Any thoughts?