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Gelcoat Crazing
For the past (too many to admit) years I have been working on the completion of a Corbin 39. Recently I have noticed that the deck gelcoat shows patterns of small cracks in various locations. These do not seem to be associated with internal bulkheads or other sources of potential stress. Since the boat has not been moved and is well supported, I discount the possibility of stresses from being lifted or moved. There is no evidence of settling or shifting. Also, there are no other sites of cracking, shifting or any signs of stress.

The deck of this boat is fiberglass covered 1/2″ plywood, about 1/8″ of FRP on top and bottom of the wood. Any ideas what may have caused the relatively sudden rash of crazing? How can I tell if the cracks are of structural importance without performing major deck surgery? If they are not currently a structural problem, what is necessary to prevent them from becoming one?

Lou Lieto
Burton, West Virginia


Congratulations on nearing the completion of your Corbin 39. Its good looking, strong and seakindly.

The deck crazing you describe is unfortunate. It is not an immediate threat, but will be time consuming to repair. And we do believe it should be repaired at some point.

One function of gelcoat is to keep water away from the underlying laminate. Being slightly porous, it isn’t perfect at this job. Cracks all the way through the gelcoat allow water to directly contact the laminate. Again, water on the laminate isn’t going to cause any immediate problem, but over time water could work its way into and through the laminate. You definitely don’t want the plywood to get wet. Ultra thin hairline cracks obviously pose less of a threat than cracks into which you can stick a knife blade.

Deck crazing can be caused by a number of things, including, as you mention, stress loads. Other causes could include improper application at the time of manufacture (too thin, too thick) or incorrect mixing of the parts, making it unusually brittle. Yet another possible cause could be expansion and contraction of the plywood core. And though weve both discounted stress, 1/8″ of glass on the top seems a bit on the thin side, so perhaps walking (jumping?) on the deck has induced stresses of which youre not even aware. A marine surveyor could probably give you a more accurate understanding of the cause.

The usual repair of crazed gelcoat involves opening up the crack. Some old books show using the V-shape of a can opener. This is crude and risks chipping large areas of gelcoat. A Dremel tool is often used. Its laborious, routing out all the cracks, but unless the cracks are widened, you can’t fill them with epoxy and any of several fillers, such as one of the WEST System brand fairing compounds.

Once routed and filled, the problem then is of making them visually disappear as the filler and deck will be different colors (unless youve used a coloring agent and are VERY GOOD at matching colors!).

Another possibility is to fill the cracks with Capt. Tolleys Creeping Crack Cure. Defender Industries (800/628-8225) sells a 2-oz. bottle for $8.95. We havent tried it.

At this point, youre probably looking at painting the entire deck, or covering non-skid areas with a non-skid pad like Treadmaster.

If the boat were ours, and the crazing does not entirely penetrate the gelcoat, does not cover the entire deck and is not an eyesore, wed leave it be for now. If the above are all true, however, wed start thinking about what color you want to paint your new deck.

Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida.

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