As­sess Your Gel Coat Cracks

Passage Maker - - Contents - Steve Zim­mer­man

Al­most all fiber­glass boats have at least a few cracks in the gel­coat and some boats look like a hard­boiled egg shell that has been rolled on the coun­ter­top. These cracks tell a story worth fol­low­ing. They might point to some stress that oc­curred the day the boat came out of the mold. Per­haps the crack re­sulted from a heavy tool fall­ing on it. Maybe the struc­ture lacks the stiff­ness needed to re­sist bend­ing and flex. Learn­ing how to read the cracks will help you de­cide if they should be re­paired and if so, how.


Gel­coat refers to the pig­mented resin on the sur­face of a fiber­glass boat. Gel­coat serves two pur­poses: it seals and pro­tects the un­der­ly­ing lam­i­nate from mois­ture and UV rays, and it helps sell boats at boat shows. Un­der a mi­cro­scope, glass fibers look like evergreen trees with branches and nee­dles pok­ing out of the resin ready to wick up mois­ture. Gel­coat seals the fibers and helps to keep mois­ture out. Dif­fer- ing for­mu­la­tions seek the op­ti­mal bal­ance of color re­ten­tion, hard­ness, mois­tur­ere­sis­tance, flex­i­bil­ity, and ease of re­pair. Ap­pli­ca­tion pro­vides another chal­lenge: too thin, the gel­coat will wear quickly and al­low the un­der­ly­ing lam­i­nate to show through; too thick and the gel­coat will craze like crazy. The dif­fer­ence be­tween too much and too lit­tle can be mea­sured in thou­sandths of an inch. Given all of these vari­ables and the de­mand­ing en­vi­ron­ment formed by ex­tended ex­po­sure to sun, salt, dirt, and move­ment, we should not be sur­prised if some cracks de­velop. It would be a mis­take how­ever, to quickly dis­miss cracks as mi­nor an­noy­ances.


Im­pact Cracks that ra­di­ate out from a cen­ter, like an as­ter­isk, usu­ally point to im­pact, such as drop­ping a winch han­dle or the crown of an an­chor. Im­pact cracks re­sult from a sin­gu­lar event and al­though they should be re­paired, at least you know that these cracks do not nec­es­sar­ily point to an un­der­ly­ing struc­tural prob­lem.

Stress: These cracks of­ten ap­pear early in the life of a boat and may ap­pear in tight cor­ners, where stresses tend to con­cen­trate. Cracks lo­cated at or near hard or tight cor­ners might oc­cur when a part is re­moved from a mold, or they might de­velop soon af­ter. Like im­pact cracks, these

usu­ally point to a one-time con­di­tion.

Flex: Cracks caused by flex­ing usu­ally run par­al­lel to each other. You might find this pat­tern where the side deck meets the cabin side, with cracks run­ning side by side and roughly par­al­lel. These cracks in­di­cate a weak­ness in the struc­ture, al­low­ing the deck to hinge with re­spect to the cabin side. This con­tin­ual move­ment even­tu­ally breaks down the gel­coat, lead­ing to cracks. Even worse, the move­ment can fa­tigue the un­der­ly­ing lam­i­nate. Un­like im­pact and stress cracks, flex­ing in­di­cates an on­go­ing sit­u­a­tion and will re­quire more than sim­ply re­pair­ing the gel­coat.

Craz­ing: Cracks in ran­dom pat­terns ap­pear­ing in many ar­eas, sug­gest­ing ap­pli­ca­tion er­rors. These can in­clude prob­lems with the prod­uct for­mu­la­tion or its ap­pli­ca­tion, usu­ally from ap­ply­ing too much ma­te­rial. These cracks tend to ap­pear all

over with­out ap­par­ent rhyme or rea­son. They might tend to be more abun­dant in ar­eas that have some stress or flex, but will also ap­pear else­where.

On older boats (25 years or more), gel­coat some­times crazes in short, ran­dom

cracks across an en­tire sur­face. In these cases the gel­coat has out­lived its ser­vice life.


Re­mem­ber that, in ad­di­tion to the aes­thetic ben­e­fits, gel­coat serves the im­por-

tant func­tional role of keep­ing mois­ture out of the lam­i­nate. Cracks cre­ate a chink in this ar­mor. Over time, wa­ter will wick into the lam­i­nate. At a min­i­mum this might lead to el­e­vated mois­ture read­ings dur­ing a pre-pur­chase sur­vey. For boats stored in north­ern cli­mates, the freezethaw cy­cle might lead to wors­en­ing of the cracks. For these rea­sons, the cracks should be re­paired.

Gel­coat cracks can­not be sealed by coat­ing over the top of them. The cracks will ex­pand and con­tract with tem­per­a­ture changes and the boat’s move­ment. The coat­ing ap­plied on top of the cracks will soon crack and you will be back where you started. In or­der to achieve an ef­fec­tive long-last­ing re­pair, the crack must be widened us­ing a Dremel tool or sim­i­lar. Once the crack has been opened, it can be filled with gel­coat tinted to match. Af­ter wet sand­ing and buff­ing, the crack will be gone, but match­ing the gel­coat can be a mad­den­ing chal­lenge. Un­less you have a new boat, the gel­coat will have aged and changed in color, more so for dark colors like blue, red, and black. Even the cor­rect gel­coat from the builder will not per­fectly match aged gel­coat. A skilled crafts­man can tweak the color with tint and come very close, but if you are ex­pect­ing per­fec­tion you might be dis­ap­pointed.

For re­pairs while cruis­ing or for doit-your­self re­pairs, a prod­uct mar­keted as Hair­line Fix by Mag­icEzy does a rea­son­able job. This prod­uct has been for­mu­lated to pro­vide the vis­cos­ity needed to flow into the crack, and the ad­he­sion and flex­i­bil­ity re­quired to re­main in place. Color choices are lim­ited and might not be a great match, but in most cases it should be close enough. You will be keep­ing the wa­ter out and the cracks won’t be vis­i­ble un­less you go look­ing for them.

Ei­ther of these so­lu­tions will work

for im­pact or stress cracks, but cracks from flex, which in­di­cate an un­der­ly­ing prob­lem, must be fixed be­fore ad­dress­ing the gel­coat. These ar­eas will re­quire ad­di­tional lam­i­na­tions of fiber­glass to pro­vide the stiff­ness needed to re­duce the move­ment that caused the cracks. Iden­ti­fy­ing the cause and cre­at­ing a so­lu­tion re­quires some­one with knowl­edge and skill work­ing with com­pos­ites.

For cracks caused by ap­pli­ca­tion er­rors or age, another level of re­pair will be re­quired. When the deck and cabin sur­face suf­fer from hundreds of ran­dom cracks, re­pair­ing each one be­comes un­fea­si­ble. At this level of sever­ity the gel­coat must be ground off. The sur­face can then be sealed with epoxy primer and painted with one of the two-part lin­ear polyurethane paints. Al­though ex­pen­sive, this ap­proach will add value to your boat and will re­tain a high-gloss fin­ish for many years. One more ad­van­tage: wax­ing will no longer be re­quired.


If you have al­ready dealt with gel­coat craz­ing then you know fiber­glass isn’t all it is cracked up to be. Os­motic blisters, wet cores, and fail­ing gel­coat serve as re­minders that while com­pos­ite boats don’t rust and don’t need caulk­ing, they do re­quire a watch­ful eye. Despite these is­sues, well-built fiber­glass boats can pro­vide decades of safe cruis­ing. Gel­coat cracks have a story to tell, and we should lis­ten. Stay­ing on top of these trou­ble­some cracks can help you avoid more se­ri­ous and costly re­pairs later.

TOP: The in­ter­sec­tion of the flex­i­ble flat deck and stiff ver­ti­cal sur­faces causes crack­ing. ABOVE: The star or as­ter­isk pat­tern usu­ally re­sults from im­pact.

The builder ap­plied too much gel­coat to this boat, re­sult­ing in a high num­ber of cracks. Be­fore a crack can be re­paired, it must be opened up with a grind­ing tool.

The gel­coat has been re­moved from an area of hair­line cracks. Milky white hues in­di­cate mois­ture in a fiber­glass lam­i­nate. The prom­i­nent white ver­ti­cal lines show el­e­vated mois­ture where the cracks ap­peared and the gen­eral white cloudi­ness shows dis­per­sio

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