blisters, boat hull that is
The type of blistering I think you are talking about is caused by osmosis..Osmotic blistering.It is caused when water inters through the gelcoat and reacts with the glass resin. It becomes acidic and damages the surface by expanding. It is more likely to occur in warmer water temperatures and on older FRG hulls that were built before barrer coats were sprayed after the gelcoat and before layingup. Check with the construction method of your hull.