We just had built a in-ground white plaster pool that has several black streaks on it. I noticed that when the plaster guys were finishing up the plaster they used a bristle brush that was leaving black marks on the plaster when they used it. Most of them they scrapped up to remove, but some were I guessing were plastered over and when we brushed for the first week, they showed up again. Aside from draining the pool and re-plastering, any other suggestions we can do? Thanks.