You have just finished hauling your boat for the winter. You have blocked her, pressure washed the hull, cleaned the topsides, and are about to button her up for the long winter's nap. One last stroll around her, gazing at the long lines, the sleek hull...sleek! eek! MY BOAT HAS THE CHICKEN POX! Don't worry, it's not really chicken pox, it's blisters, and, as in most early childhood diseases, it is not terminal. But it does take some understanding of what causes blisters and what to do when your pride and joy shows up with them.
Water being absorbed through the gelcoat and fiberglass laminates in the hull causes blisters. Once thought impervious to water, it has been discovered that constant contact with water can cause it to be absorbed through the gelcoat from the outside and through the exposed laminate on the inside of the vessel (from standing bilge water). The water absorbed causes changes in the physical makeup of the hull and reacts with the resins, which build up residues and finally raise blisters by increased pressure on voids between the laminate and the gelcoat. This is what you see on the hull. This phenomena usually starts occurring, although it may occur earlier, from the fifth to tenth year after the manufacture of the boat. It occurs when boats are left in the water for long periods of time and happens more frequently in fresh water.
Once blisters start to show up, it is not necessarily time to panic. If you have a good antifouling paint on the hull and you only see a few blisters but no cracks in the gelcoat, you need not be too concerned at this point. However, if the number and size of the blisters increases and you start to find cracks in the gelcoat itself, it is time to jump into action to repair the problem.
The first step is to remove the antifouling paint and get to the blister problem first hand. The next step is to open up the blisters to allow them to "bleed" the excess water. You may find areas where you can't actually see the blister but you do see water "weeping" from the gelcoat. Take a pocket knife to start the opening of each of the blisters and weeping areas and then grind down to solid laminate. Once you have opened these voids you will be rewarded with a very distinctive aroma which is generated from the foul residues. (Remember, water may also penetrate from the inside so make sure the bilges are dry.) Once all the blisters have been ground to solid laminate and the entire void exposed, allow the hull to dry for as long as possible. This may require a month or two or more. Older boats or boats in cold, damp conditions will take longer than newer boats or boats in hot, dry conditions.
