I'm pretty sure it was just a base coat of flat latex...the original that was put on when the house was built. I will be repairing with premium paint though.

As far as that is concerned, it is still a consideration. I could have saved myself a LOT of grief if I had just installed a cheap $10 piece of thin acrylic mounted to the wall. Where my big tank was, I also noticied it bubbling up, but the walls in that part of the house have since been painted, and it does/has not happen(ed) since.