My experience is with fiberglass boats but have some suggestions. The blisters need to be ground down. It is likely just the gelcoat that is blistered but blisters can form between the fiberglass layers as well. We will assume it is just the gelcoat. Anyway, you grind off the raised blisters and feather the edge of the ground area. You can then fill the defects with epoxy that has a fairing filler added. This thickens the epoxy so it is more putty or spackle like so it can be spread in the defects. Then you sand the repairs smooth and prep the rest of the shell and then paint with the aforementioned paint. This would be time consuming but pretty easy and you could do it yourself. I use West System epoxy for boat repairs and they have pamphlets that clearly outline how to do repairs like this.

The other option would be to remove all of the existing gelcoat and then redoing the entire shell with new gelcoat. This would be a big job obviously.