Beating Boat Blisters

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If you’ve kept your boat stationary this winter, we hope that you’ve winterised her properly. Luckily spring is nearing slowly but steadily and it’s almost time to take the covers off! When you do take those winter covers off on your first day of weather-permitting spring boating, we sincerely hope you don’t find the dreaded ‘pox’ along your hull. But if you do, don’t panic, blisters aren’t the end of the world and can be mended fairly easily.

First, let’s explain what blisters are. Essentially, blisters form due to water being absorbed through the gel coat and fibreglass of your hull. The unwelcome phenomenon of blistering is usually found on older boats – from about five to 10 years after production and particularly in boats that are left in water over extended periods. When your boat is in constant contact with water, moisture will eventually find its way through the gel coat on the outside and through the exposed laminate on the inside of the vessel.

The absorbed water 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 gel coat. It is important to note that all gel coats, resins, and coatings used in boats are permeable, i.e. all let in moisture to some extent. Even epoxies, the most waterproof resinous coatings available, are permeable. It is imperative to have a solid ‘resin rich’ laminate, as free of air voids as possible behind it. Without a good laminate you’ll be dealing with blisters far more often. Blisters are one of the most common deterrents of potential boat buyers and with a little time and effort you can significantly increase the resale value on your boat by repairing them and taking steps to prevent them from popping up again.

How to go about repairing blisters

If there are a few blisters visible on your hull but no cracks in the coating, your problem isn’t too severe and can be fixed fairly easily. If you find cracks in the hull and the blisters seem to enlarge and increase in number you should start taking steps immediately. Cracks can indicate structural problems and you might want to consult a reputable manufacturer or dealer about the problem. However, you can repair surface blisters by yourself without paying the ludicrous amounts that professionals will inevitably charge to do it for you.

First, make sure your bilges are dry, as water may also penetrate from the inside. Next, remove the antifouling paint on your hull to expose the blister in question. Using a pocket knife or other tool, open the blister to allow the water to bleed out, and then grind down the coating to the laminate. Once you’ve done this to all the blisters with the voids exposed, allow the hull to dry fully. This may take up to a month or two, depending on weather conditions, etc. Older boats and boats that have spent more time submerged in water will take longer than newer or less used boats.

To test whether the hull is dry, tape a length of clear plastic over one of the grinded voids, making sure the edges are sealed tightly and leave it there over night. If the plastic sheet is without condensation then your hull has dried sufficiently. There are a great many products available on the market to counter and repair blisters. It is advisable to consult your manufacturer as to what product will be best suited to your hull. If you’re hesitant to ask your manufacturer, drop Leisure Boating a line on our Facebook page (@LeisureBoating) with your make, model and problem and we’ll find out which products will solve your problem best. Nevertheless, once you’ve found the right repairer it is imperative that you follow the product’s directions carefully when repairing your hull.

Once the blister voids are dry, follow the directions on the product and fill the void with whatever epoxy or fairing compound you decided on. Give it enough time to dry properly and then sand down the area until smooth and back to the original lines of the hull. The next step is to select the best antifouling paint for your hull; buying the wrong type will have you right back to scratch before long, so make sure and speak to other boaters before purchasing your antifouling paint.


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