More than 30 years ago, the use of fiberglass and resin to build boats revolutionized the marine manufac­turing process.

Renown for high strength, dura­bility and presumed low mainte­nance, fiberglass was deemed to be almost indestructible. In the early 70s, it was rumored that fiberglass boats were being devoured by "polyestermites." Those in the know compared these glass-eating bugs to the teredo worm, the nemesis of wooden boat owners. It was discov­ered to be a hoax; although perhaps the mites creators forewarned of what we now call osmosis.

A fiberglass hull is composed of two layers. The base layer, or poly­ester resin-reinforced fiberglass lami­nate, is covered by gelcoat, a pig­mented polyester resin. All fiberglass laminates are semipermeable, mean­ing they allow water to pass through the outer layers. Osmosis is the process of moisture seeping into the fiberglass laminate, either through the gelcoat layer or internally from the bilge areas. As moisture migrates into the laminate it fills voids, becomes trapped and creates an acidic blister fluid. Still seeking to dif­fuse equally throughout the substrate, the water between the laminate and outer surface places pressure on the gelcoat. The resulting blemishes or blisters that form between the lami­nate and the gelcoat affect the appearance and performance of the boat. As excessive moisture is the culprit, blistering usually only occurs below the waterline.

When blisters are left unchecked they will progressively extend deeper into the laminate and affect the struc­tural integrity of the hull. The term used to describe the chemical reac­tion between polyester resin

lami­nates and water is hydrolysis. Over time, blister fluids attack the resin in the laminate, severing the chemical bond between the resin and lami­nate. As it progresses, the bottom becomes spongy and delaminates. Unfortunately, this condition is not reversible but can be repaired once it has started. Any hydrolyzed lami­nate on a boat must be removed and the bottom relaminated. This usually requires the services of a profession­al.

Regardless of the quality of con­struction, osmosis blistering and water absorption into the laminate occur in most boats sooner or later. It's the nature of the beast!

Variations in resins, catalysts, humid­ity and workmanship in the manu­facturing process all determine when and to what extent gelcoat blisters appear.

Repair Techniques

Sealing the gelcoat from moisture with an epoxy-based coating helps to prevent osmosis. If your boat develops small and shallow blisters you can repair it yourself. The best time to spot blisters is when the boat is first hauled out of the water. Immediately mark the blisters with a waterproof pen. Blisters may begin to disappear within a few hours but will reabsorb water when the boat is launched.

Osmosis may not be evident under several coats of antifouling. Scraping or sanding may reveal bumps or crescent-shaped cracks from the size of a match head to a quarter. Before beginning to remove the antifouling paint, wash and clean the hull area being repaired. Removing all of the surface contami­nants -marine growths such as algae, barnacles, sea grass, etc. ­is the first thing that needs to be done. Use an acid-based cleaner to quickly dissolve any growth and grime.

Next, remove the bottom paint from the affected areas using a 24- to 36- grit abrasive. This should take off 60% to 70% of the bottom paint. Some abrasives are specifically designed for this process. They utilize a special coating process to prevent them from loading on soft bottom paints and fairing compounds and give an increased cut rate.  Use a finer grit (60 to 100) to remove the remaining coating. Make sure you take the proper safety pre­cautions before sanding. When removing bottom paint or sanding fiberglass and gelcoat always wear protective clothing, gloves and a face respirator. Some manufacturers do not recommend using chemical paint strippers to remove bottom paints. Such products facilitate paint removal but may damage gelcoat or the polyester resin in the laminate. Another option is to use Peel Away, a non-methylene-chloride stripper that removes multiple bottom paint coatings from fiberglass, wood, steel, or aluminum.

Where a bottom has small, isolated blisters it is not necessary to remove the gelcoat. Clean the surface with a dewaxer or solvent wash. Open each blister individ­ually then fill and fair the blister cavities followed by an epoxy coating. Use 3 a countersink bit on a ~ variable-speed drill to open blisters. Wear pro­tective clothing and eye protection (goggles or full face shield). The acidic fluid may be under as much as 200 psi of pressure.

If the bottom is covered with small or large blisters that go into the laminate, you'll need to remove the gelcoat surface down to the fiber­glass laminate and at least 5cm (2") above the waterline. This is accomplished by either sandblasting, peel­ing or grinding. Sandblasting with sand or other mediums will remove the gelcoat and open any blister cav­ities but must be done by a compe­tent operator -in the wrong hands, sandblasting can blast a hole right through the laminate. Irs effective but can embed particles in the lami­nate and leave a bumpy finish.

Gelcoat peeling is the most efficient method of removing gel coat. It leaves a clean, smooth surface, but because it requires expensive hand-held or robotic equipment, must be done by a contractor. Grinding is painstaking slow and messy work but the most common solution for do-it-yourselfers.

Use a grinder fitted with a soft back-up pad and 24- to 36 grit discs and grind until you reach the lami­nate layer. Hold the grinder at a low angle to avoid gouges. Be careful not to cut into the laminate. Often grinding removes all blistering occur­ring between the gelcoat and skin coat, the first layer of mat. If you can still see light-colored circles of delam­ination, you'll need to remove the damaged laminate. Using a 7.6cm (3") 24- or 36- grit grinding disc on a drill attachment, grind out the cont­aminated laminate area, forming small craters until all signs of delami­nation are removed. Gougeon Brothers recommend "sounding" the hull with a rubber mallet to detect any laminate degradation. Wet or delaminated areas will sound dull or flat; dry, solid laminate has a sharp sound. If there is evidence of delami­nation or blistering above the water­line consult a surveyor.

Scrub the area with detergent and flush out with freshwater to remove any contaminants. Allow the area to dry to remove all moisture from the bottom. Drying could take anywhere from two weeks to two months or possibly longer depending on the severity of the blistering. The more blistering, the more water the hull has retained. Removing all of the moisture from the hull prior to repair­ing and recoating will minimize the possibility of blisters reoccurring. Infrared heat lamps or tenting the hull area with plastic sheeting will speed up the drying process. In the latter case, wrap the bottom in plastic, held in place above the waterline with tape and weighted at the bot­tom to form a tight seal.

Dehumidifiers and small fans to keep the air moving are placed inside the tent. Keep the bilge dry and well ventilated to prevent moisture from entering the laminate from the inside. If the boat is stored inside, use heaters or fans to force dry the sur­face.

Check the moisture content of the fiberglass laminate with a mois­ture meter before applying any coat­ing. Test the hull both above and below the waterline in several areas every two to three feet apart. Mark the spots on the hull where you take the readings and every few weeks again check the moisture content. A meter reading of 2% or 3% is recom­mended. Another method to deter­mine hull dryness is to lay 30.4cm (12") squares of clear plastic sheet over the surface and seal all edges with tape. Leave for at least 24 hours. If condensation forms, the hull requires further drying. Remove the plastic, wipe the area and plastic dry and repeat until the plastic remains dry.

When thoroughly dry, wash the bottom again with freshwater. Scrub the surface with a 3M Scotch Brite pad to remove any surface contami­nates leached out during drying such as glycol, a liquid component of polyester resins that unlike water, does not evaporate. Some manufac­turers recommend pressure washing the hull with hot freshwater every three to four weeks during the drying processing to remove all glycol con­tained in the laminate. When the sur­face has thoroughly dried, apply a sealer coat to cover the exposed laminate surfaces. These sealers are typically low viscosity epoxy coat­ings designed to seal voids and pin holes prior to applying a fairing compound and barrier coatings. Follow the manufacturer's directions. Epoxy coatings developed for blister repair include many epoxy resin sys­tems (West Systems, East Epoxy and others), Interlux's Interprotect System and Sikkens Epoxy GP System.

The surface is now ready for fairing. Fairing is the process of mak­ing the surface flat and smooth again. Any surface bumps or dents in the hull will result in a decrease in performance and appearance. Apply the fairing compound to fill the exposed cavities. Use large putty knives and battens to apply a smooth layer of filler. Several thin coats are better than one thick coat. Don't use a polyester fairing com­pound below the waterline. Epoxy or vinylester work best here. Again, fol­low the manufacturer's instructions. When dry, sand off high spots, remove dust and refill low spots. Be sure there are no pinholes or glass strands protruding through the filler.

Once the fairing compound material has dried, proceed to sand or "rough shape" with 36-grit sand­paper or disc to knock down the material quickly. Don't sand too much or you will have to apply more fairing compound to your low spots. Finish sanding with 80 grit followed by 180 grit to refine the surface for painting. Follow manu­facturer's instructions for finishing details.

Multiple coats of a recommended epoxy barrier coating followed by a primer and antifouling paint are now applied according to manufacturer1s instructions at this time. Wash the hull with the recommended solvent before applying primer.


Is osmosis preventable? It's estimated that one in four fiberglass boats will get gelcoat blisters in its lifetime. The obvious answer is yes, there are preventative measures you can take to slow down and delay this occurrence. The first line of defense is your boafs gelcoat. Apply a barrier coating to new hulls before commissioning. Other protective measures such as dry storage, frequent spot repair of defects in the gelcoat and additional hull coat­ings will help slow down and delay the possibility of blis­ters forming.

Epoxy coatings applied on a sound, dry hull provide a durable, water-resistant barrier; however no system is foolproof. Even epoxy coatings allow some absorption. The key to a successful repair is to remove all of the dam­aged laminate, thoroughly dry the hull and correctly apply the barrier coats. To reduce the possibility of blis­ters reoccurring keep the bilge as dry as possible and the boat well ventilated. Install additional vents to eliminate condensation. Solar-powered vents offer an efficient and affordable means of increasing air flow. Finally, check the barrier coat annually. If scraped and dinged, repair and recoat promptly.