Grey spots

I’ve got every spreading grey spots on my plaster in ground pool. 13,000 gallons.

it started about 3 months ago wit the first spot I noticed, about the size of a dime.  Now it’s the size of a quarter and I have about 10-15 more.

I felt it, it have no different feel from the rest it the pool. It just feels like plaster.  I’ve tried pumice stick, wire brush, and even used my power washer on it.  Shocked (up to 12). Didn’t even touch it.

The folks at the local pool stores, tried 3 different one, had no idea.

Metals, fertilizer, black mold.  But no real idea. Do YOU have an idea?

Usually, gray spots are a plaster defect. This could be treated by acid washing or sand blasting the surface.

There is a chance that it could be cobalt or iron. If that’s the case, you could use a stain ID kit to identify the metal. We recommend the Jack’s Magic Stain ID Kit. It comes with three chemicals and instructions on how to use them on the stain. A single chemical or a combination of two may remove the stain. Then you would order a larger quantity of the chemical(s) that worked.

Just to rule out the possibility of it being algae or mold, you can rub a chlorine tablet on one of the spots (wear a glove). If the spot lightens up or goes away, it’s organic and can be treated with chlorine and algaecide.

I’ve tried pressure washing.  That had zero effect.

I’d try the chlorine tablet first to rule out anything organic and then I would try the Stain ID kit.