So is it a fiberglass wall & gunite bottom?

Bubbles behind paint are 1 of 2 things. A contaminant left in the surface (including moisture) or the paint was applied in 90+ temps in the sun. I've experienced both.

There is an epoxy paint that will go over any type of paint. We use it and it holds up very well. That said, I still hate painting pools.

Lew me know about your pool construction. I don't mess with fiberglass but can help with the other stuff.