Why is my plaster stained?

[djm181]

I don't know what I'm looking at in this picture but a lot of my pool plaster has this sort of discolouration. The plaster is fairly old now but this is a recent development. Can someone tell me what this greenish yellow stuff is and how to treat it? It doesn't feel slimey and won't come off with brushing.

17k gallon plaster pool in ground.

 

[Texas Splash]

Welcome to TFP! It has an organic look to it. Can you post a full set of water results for us? That may help give us a clue as to what's going on. How does the rest of the water look?

Also, don't forget to update your signature with all of your pool and equipment info.

 

[djm181]

The water is crystal clear. Here are my test results (with a Taylor 2006 kit)

FC 7.8
CC 0.0
pH 7.4
TA 70
CH 390
CYA 50
Temp 82F

Where this stuff is is under a tree on one side of the pool. However, the steps on the other side of the pool are completely unshaded - the second and third steps have similar discolouration, but the top step is nice white plaster.

I use cal-hypo tabs in my skimmer basket for chlorine, supplemented with liquid chlorine as necessary.

 

[kadavis]

Time to replaster for the original post, the other looks like a serious black algae problem. If you have one or go buy chlorine tablet and rub directly on the black spots and check next day

 

[djm181]

Time to replaster for the original post

Yeah, I had come to this conclusion myself too. Out of curiosity, what is happening here as the plaster ages to cause these green marks?

 

[Texas Splash]

Out of curiosity, what is happening here as the plaster ages to cause these green marks?

As the plaster deteriorates, it exposes the material underneath which is more pourous and susceptible to algae getting a foothold, rooting, etc. It also makes aggressive brushing more difficult as you don't want to make weak plaster areas worse.