Osmotic Blistering

Osmotic Blistering is a condition found in most fiberglass boats. Blistering occurs when water penetrates through the gel coat and mixes with water soluble materials found in the fiberglass layers beneath it. The seawater then becomes a solution that will try to equalize with the seawater outside the boat. As the solution has now grown molecularly, it cannot back out of the passage it entered, it builds up pressure as it tries to escape and then it explodes, resulting in blister domes which appear on the outside of the hull.

The repair procedure is as follows, first a technician removes all of the anti-fouling paint, gel coat and any damaged fiberglass by peeling or grinding the underwater body. The underwater body is then scrubbed with a solution of TSP and water to remove any stray contaminates. Next, the boat is tented off tight around the bottom and a dehumidifier is run in the tent to keep the air as dry as possible. Once the hull bottom has dried to an acceptable level, an epoxy barrier coating system may be applied. The drying process may take from 2 weeks to 12 months.

Epoxy barrier coatings have proven to be very effective in repair and prevention of blistering. Epoxy is very dense and tight making it non-conductive to water penetration. In cases of severe blistering, an additional layer of fiberglass cloth may be applied with the epoxy coatings to replace any fiberglass removed during peeling and grinding and to add strength to the total system.

Osmotic blistering usually starts as a nuisance/cosmetic problem. However, blistering left untreated can cause a very serious condition resulting in de-lamination of the fiberglass hull bottom, very extensive as well as expensive repairs or in extreme cases, total loss of the affected boat.

Right: Here Gary is removing the bottom paint to be able to determine the extent of the osmotic blistering.

                        Step 1 we start with a haul out and block. Power washing would be optional and situational prior  to peeling. Step 2 is coatings removal via a mechanical hand held gel coat peeler. Step 3 is protective coverings for dust containment and protection of the boats’ topsides. Step 4 is soft padding and disc grinding as necessary to reach areas in-accessible  to the peeler and to blend in peeler lines. Step 5 is power washing and scrubbing with various compounds to remove salts, glycols or other contaminates trapped in the laminate. Step 6 is metering and monitoring while drying. Step 7 is the base coat of epoxy which is a clear sealer type epoxy common to most good systems. Step 8 is faring. This is the most demanding and labor intensive step of them all. 

            Our feeling is that the hull must be returned to as near a “as new” degree of fair as possible. To accomplish this, technicians will spend considerable hours with trowel able compounds and long board sanders. This is one of the key steps that sets Cap Sante Marine above other blister repairs. We have a special skill and understanding that delivers better results quicker. 

            Step 9 is barrier coatings. We use an air assisted airless type spray unit to apply epoxy barrier coatings. The results are better application and much thicker mileage. Please note we use industrial type epoxies not available to the general public. This along with spray application is a step above standard boatyard practices.  Step 10 is anti-fouling coatings. Type and brand may be of owners choice. We recommend ablative type anti-fouling airless applied in two coats of opposing colors. Step 11 is shifting stands and blocks and repeating the steps as necessary. Step 12 is strip masking and clean up. 

            Other fees you may expect are general shop supplies, EPA  hazardous materials disposal fees, mast and rigging work as necessary to get you into a building, power usage during dry time, minor mechanical as necessary after a long lay up, hull and topsides detailing if desired and, of course, your final launch. We do not charge lay days for jobs in progress. 

            These are the details in a nutshell.  We offer a conditional 5 year warranty. We also offer a perfunctionary  job allowing the vessel to be returned to service quickly, no warranty is offered on that job. Also, we have done jobs even more involved than our premium repairs. In some cases we have double peeled severely  blistered hulls and re-laminated with epoxy prior to recoating.  

Gary Harden

Cap Sante Marine