Gelcoat

Repairing nicks, scratches, cracks, and surface crazing
Gelcoat is made of polyester resin, the same stuff used to build the hull of your boat, except that it has none of the fiberglass embedded in it. Your gelcoat may be pigmented and it may also have additives to protect it against destructive ultraviolet rays. It may even be formulated to be more waterproof than normal, and to be more resistant to abrasion. But basically it’s just plain old polyester resin.

Cross-section through a fiberglass hull showing typical sandwich construction with a closed-cell foam (top) or balsa (bottom) core. The fiberglass laminate commonly comprises alternating layers of mat and woven roving. Often there are two layers of mat immediately beneath the thin gelcoat to prevent “print-through” of the coarse weave of the underlying roving.

It is normally applied in a very thin layer—about 20 mils (0.02 in. or 0.5 mm)—and when it is accidentally applied in thicker layers during construction, its brittleness causes it to crack. For these cracks and others caused by impacts or excessive strain, matching gelcoat repair kits are available at marine hardware stores. They are also good for fixing crazing, scratches, and minor gouges.Cracks should be opened up into a deep V by drawing the corner of a screwdriver or a can punch along them. That way, the repair gelcoat can penetrate properly and not merely bridge the gap.Most repair gelcoat needs to be protected from the air after it has been smoothed into place; otherwise, it will not cure hard. Seal it with plastic wrap and tape or spray it with a PVA (polyvinyl acetate) curing agent.If you plan to paint over the repair afterward, you can use epoxy resin or epoxy putty instead of repair gelcoat. The epoxy has the advantage of remaining flexible when cured. Epoxy paints designed as surface finishers do a good job of repairing crazing on old gelcoat. Remember, though, that you cannot then finish off with gelcoat because, whereas epoxy will cling tenaciously to matured polyester, the polyester gelcoat will not bond to an under layer of epoxy. You must paint over epoxy.New gelcoat eventually oxidizes and becomes dull and chalky, which is especially noticeable in dark colors. You can renew the surface gloss by buffing or rubbing with a fine compound. It is not necessary to wax the gelcoat, but many boatowners do so anyway—not only for the brilliant sheen, but also because they believe it protects the gel-coat from oxidation. If you’re planning to paint the topsides or deck in the future, however, don’t use a silicone wax—it is extremely difficult to remove and every tiny bit must go before you can paint. Use a good carnauba-based wax, which is easily and thoroughly removed with the solvent toluene.See also Epoxy; Fiberglass; Paints.

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