Fouling paint won’t do a thing for osmotic blisters anyway, don't bother. Paint isn't waterproof, and believe it or not, neither is fiberglass unless it’s laid up with epoxy or vinyl-ester. Despite their cost the majority of towboat builders continue to use only cheap old Polyester resins with no infusion let alone post cure process. Perhaps its because they intend their products to be trailer boats suited only for day use. If you really want to stop osmotic blistering you will need to apply a barrier coat like System 2000-2001. It is basically a several coat lay-up of epoxy paint.
It comes in gray or white and will require you abrade the hull.
Blisters are born in the mold. All you need is a tiny drop of moisture in the matrix to contact some un-dissolved solids and the process begins. That is why most upper tier real boat builders will use vinyl-ester for the skin coats and first few mat layers before switching to polyester for the bulk of the laminate. Very expensive hulls, price-per foot average of say MC or SN warrants a full epoxy or vinyl-ester schedule. This just doesn’t happen though. The money must be being spent somewhere more important.......<<<<