What is this and how do I get it off?

[MichaelGarrison]

What are these spots?


They are not really recent. They've been appearing for the last year I think. They are dull enough that they didn't bother me until I figured out I couldn't remove them. The pumice stone doesn't take them off. Neither does the stainless steel brush. It's not black algae...I have a little of that too so I know what it looks like.

What do you think?

 

[woodyp]

Rebar bleeding in to the pool surface maybe?

 

[MichaelGarrison]

Now I didn't expect that answer.
Is that something that naturally happens for my type of pool? Is it a result of poor construction?

I'll go take a closer look when it stops raining, and see if I can tell if it is on the surface or seems to be throughout the concrete.

 

[MichaelGarrison]

It's hard to tell, but it really does look like a surface stain. The spots are also not in any regular pattern, which I would expect if it was rebar.

 

[kadavis]

Possibly calcium nodules, do they feel rough and raised above the surface compared to the surrounding area

 

[MichaelGarrison]

No they don't feel raised. I also took a pumice stone to it and nothing seemed to come off.

 

[rphpool]

Try putting acid in a small bottle that has small opening. Now invert that bottle on top of the stain for 30 seconds. I have used this method with various stains