Angling Addict is spot on . Bottom paint is there to prevent aquatic growth on the hull. A barrier coat of epoxy is what you need.
I had exactly the same problem on a Seaswirl WAC that I kept in a slip. I went to a fiberglas guy in the marina that came highly recommended. He sanded the blisters and used an epoxy barrier coat, which is like forever. Then if you desire , you can bottom paint to retard growth. Not cheap, but a permanent solution.