Looks Like Chicken Pox - AKA Blisters
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.
A good technique to test the hull for dryness is to tape a clear plastic sheet, about one foot by one foot, over the clean hull surface. Make sure all the edges are sealed with tape and adhere to the hull. After 24 hours, check to see if there is any condensation under the plastic. If not, you're ready to continue. If you find condensation you need more drying time. You can make a plastic skirt around the hull from the water line to the ground and place fans or, with great care, heaters to speed the drying process.
There are many products available to make the repairs to the blisters. If you plan on doing the job yourself, talk to others in your boatyard or marina who have had success and ask them to recommend products. It is important to follow the product directions carefully. Once you have your blisters exposed and dry, fill the voids and ground areas with the epoxy or fairing compound that you have selected. Once dry, board sand the bottom to fair in the repair and repaint with your antifouling paint. (Fair is a term used in ship building by which is meant the restoring to original shape any part of the ship's structure not damaged seriously enough to necessitate actual removal from the ship for repair. In this case it means that you want to use a large enough sanding area so the repairs blend in and are not noticeable.)
How can you prevent blisters? Well...the key is to keep the hull as waterproof as possible. Some boaters recommend the use of one of the good epoxy coatings to coat the entire hull. This can be time-consuming and moderately expensive however, it seems to cut down on the dreaded "chicken pox."
If you want to undertake this endeavor, make sure the entire hull is clean and free of antifouling paint, any other foreign materials and moisture. Sand the gelcoat enough to make sure all the shine is gone. The color should be consistent and bright. Again, using your favorite epoxy which is intended for boat bottoms, build up a generous coat. Remember, the epoxy is what gives the protection. Try to get a minimum of 15 mils of thickness. This is about five times the thickness of a coat of paint and about the thickness of typical gelcoat. Once you have completed this operation and sanded to fair the hull, reapply your favorite antifouling paint.
Blistering is the most common reason that potential boat buyers back off or make substantially lower offers. Repairing blisters once they have occurred, or taking steps to prevent them, can improve the resale value of your boat.