Why is my pool plaster turning a yellowish tan? I have initially perfectly white pool plaster and over the winter the plaster turns a slight tan color in some areas. Actually white plaster areas are a lot lighter then see in photo. I did originally surface ground this off the old plaster and was able to successfully apply new plaster with the help of Sitka Pros acrylic bonding agent and this may be because the acrylic is an organic material. But it was the worst at the bottom of the pool where leaves and I assume tannin from the leaves collected. I was able to remove most of the tan by acid washing but at the bottom of the pool I did have to surface grind with a diamond disk. In my area the warter is notoriusly hard, so could it be calcium chloride? When surface ground plaster underneath is perfectly white.
If you do have some testing history, feel free to share.