blisters in pool plaster

[tankcj]

Hello,

I am seeing small blisters about the size of a quarter on the steps of my pool. You can push down on the plaster and small bubbles will appear. In one area it flaked off exposing a 'crater' about .030" deep. What is causing this?

[Watermom]

Bumping this thread back up to see if anyone has any advice for this poster.