What are these brown spots in the plaster?

[Roger Bull]

These spots started showing up after plastering even before filling. There seems to be a source pebble in each one. If I dig it out and acid treat the area, the spot goes away. The problem is that more and more are showing up and it has been 5 weeks since plastering. PB says he has no idea. Has anyone seen this before?
These photos were taken prior to acid wash and fill but the new ones look the same. Acid does clean the stain.

 

[zea3]

Is this a new pool or re-plaster? It looks like rust stains. Have you tried rubbing a vitamin C tablet on one of the stains? If it takes it off it is rust. You may have some rebar ties close to the surface where the plaster is thin and the rust is bleeding through.

 

[mknauss]

There seems to be a source pebble in each one. If I dig it out and acid treat the area, the spot goes away.

Might be what Zea said --- or ---- did the plaster have abalone shells in it?

 

[Roger Bull]

It's a re-plaster but I'm sure it's not rebar. It seems to be a contaminant in the pebble. I had not thought about abalone shell. I have one leftover bag of the pebble mix and will check it when I get home later. What happens with abalone shells?

 

[Roger Bull]

Since acid cleans them up (temporarily) I'm starting to wonder if they are metal shavings from the mixer or pump. Will try to dig one out tomorrow to see if it is magnetic.

 

[mknauss]

What happens with abalone shells?

Abalone shells that are properly treated will not be effected. Cheap abalone shells can break down and cause stains.

Yours does look like iron. Use vitamin C instead of acid to clean them. Acid will harm the plaster finish.

 

[Roger Bull]

Mystery solved. It is caused by iron fragments. I dug out the center of a couple of the spots while holding a super magnet nearby. Each time the super magnet came up with a small piece of rusty iron less than a mm in diameter.
I'm letting the PB know now. I really could use some advice on how to proceed from here.

 

[mknauss]

I suspect you will have to wait until they show up and do as you have done. I would use ascorbic acid instead of muriatic acid to clean them up.

 

[Roger Bull]

Any guesses as to how long before they stop showing up?
I let the PB know and he has at least one other pool with the same problem. He is meeting with the plasterer tomorrow.
I told him I'd wait it out if the spots stopped appearing before the start of next season.

 

[mknauss]

All depends on how many pieces of iron got mixed into your plaster. As the plaster surface erodes over the next 20 plus years I suspect more will show up.