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Fiberglass Spider Crack Repair Done Right

Sure, there are a number of fast fixes out there for fiberglass cracks—but if you want it done right, follow these five easy steps.

Stress cracks, spider cracks, and hairline cracks in the decks of older boats are, of course, not rare. Sometimes, they’re merely cosmetic, meaning they don’t penetrate the gelcoat. Sometimes, they go deeper and threaten an underlying laminate, which in many cases is cored. Because the latter scenario may evolve into a very expensive proposition (replacing coring materials in an existing deck is a messy, pricey job best left to the professionals at your yard), it makes sense to address all suspicious-looking cracks on board your boat as soon as possible, especially in areas that feel a little spongy (probably due to coring issues) when stepped on.

Before you begin, however, a word of warning. There are numerous, inexpensive products on the market these days that purport to be easy fixes for hairline and other fractures in fiberglass laminates. I’ve personally tried two of the most popular of the lot—MagicEzy Hairline Fix and Captain Tolley’s Creeping Crack Cure—and, although certainly easy to use, neither one comes close to being a substitute for doing the job the right way, as follows:

Step 1: Figure out what caused the cracks to begin with. Do they emanate from a stanchion base that flexes? Do they spread out from a radius that vibrates or twists in a seaway? Do they surround an area that feels just a little spongy, as already noted? Think about it.

Step 2: Then toughen the laminate that contains the crack or cracks by solidifying it, either by thickening with more fiberglass, replacing coring materials, or adding structure. If you don’t do this, new cracks will most likely appear, probably not too long after you’ve just finished fixing the old ones.

Step 3: Now, once you’ve removed all hardware, toerails, and other paraphernalia that may clutter or interfere, break out a Dremel tool with a conical bit. Then follow each separate crack with the Dremel as if you were wielding a pencil. Use the highest speed possible, since low speeds may cause the bit to catch or swerve damagingly from your intended path. And let the Dremel do the work—several shallow passes will produce a cleaner, more accurate job than going for a single, deep, forced march.

Begin with a shallow pass with a Dremel tool. 

Begin with a shallow pass with a Dremel tool. 

Step 4: As soon as the crack has been channeled out, wipe it clean with a solvent like acetone and, using a plastic putty knife, flush-fill it with gelcoat matched to the color of the surrounding area (if the channel isn’t too deep); polyester filler (if the channel is of medium depth); or polyester resin and fiberglass strands (if the channel is very deep and wide). It’s a good idea, by the way, to mix up a couple of small batches of gelcoat to help you approximate the color you need. Also, epoxy can be used in place of polyester in some situations, although it tends to be much harder than poly and therefore more difficult to sand smooth and finish. And don’t forget to temporarily cover all gelcoat repairs with plastic film—gelcoat does not cure thoroughly when exposed to air.

Step 5: Finishing is the last aspect of a proper fix. When dealing with gelcoat, dry sanding followed by wet sanding, with ever more refined, smoother grades of paper (starting with 220-grit and working your way down to super-fine 600-grit) is advisable, followed by a good wax or polish. When dealing with a polyester or epoxy repair, numerous cracks are usually involved, and you may be looking at a paint job of some kind in the near future.


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Capt. Bill Pike is deputy editor of our sister publication Power & Motoryacht magazine.