How to repair your boat’s gelcoat

Nicks, dings, and itsy-bitsy cracks are facts of life for a cottage boat. Make it smoother with some simple gelcoat repair

By Tom CarpenterTom Carpenter

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George Groen, owner of the Boat Repair Centre near Kingston, Ont., offers guarded encouragement to those attempting a DIY gelcoat repair. “Anyone can do the little stuff,” he says, pausing to rub his hand over a series of DIY repairs on the side of the hull in his shop. “But it’s tricky. And it’s like Chinese cooking – the food’s only in the wok for seconds, but you’re preparing for an hour.” It’s that preparation, he goes on to explain, that determines whether a repair will last; so if you are considering touching up the gelcoat on your own boat, be patient. What follows is a very particular process of sanding the repair area, filling voids with multiple coats of colour-matched polyester resin (with yet more sanding between coats), then waxing and buffing to achieve a glossy finish.

While it might seem obvious to fill in the opening with more material, the problem is that gelcoat is not an adhesive; it won’t glue the two sides back together. What is more, once gelcoat sets it is brittle, and if you fill the gap, the new material will quickly crack again.

Instead, what you must do is smooth the steep-walled canyon down into a gently sloping valley. That way, when you fill the area with gelcoat, the material will have lots of surface to grab onto and no sharp edges at which to break again.

Paradoxically, in addition to being smooth, a repair area must also be rough. Just as jam sticks better to a terry-cloth towel than it does to a mirror, the gelcoat forms a far more permanent attachment to a surface that has been roughed up than it does to one that is shiny smooth. In practical terms, this means that, within strict limits, your prep work can be quite vigorous. Use a rotary tool, a screwdriver, or coarse sandpaper to rake out the extra material and create the valley. Groen calls it “ V-ing out,” and you want to end up with the edges of the opening bevelled way back. (See Fig. 1.)

Now about those strict limits — before you touch your hull, you should lay down a barrier of masking tape between where you want to work and where you don’t want to accidentally scuff your shiny finish. Mark out an area a couple of millimetres back from the edges of the damage. Once you’ve carved out your little valley, use some 120-grit sandpaper to simultaneously smooth down the topography – even the edges of a shallow gouge should be cut flat — and roughen up the terrain. Then you’re ready to fill.

Mix and match

Unfortunately, now comes the hard part, and everyone has the same benign-sounding opinion about mixing pigments into the gelcoat to match the existing colour of the hull. “It’s a bit of an art,” explains Groen. (“It’s an art,” echoes Jamie Hewitt. “It’s a little bit of an art,” adds Little.) And by this, of course, what they really mean is that it’s not a science; careful measuring alone guarantees nothing when you’re mixing colour; there is just trial and error, frustration, and a couple of warnings.

Gelcoat is a two-part system made up of a particularly dense form of polyester resin and a hardener (methyl ethyl ketone peroxide or MEK). On a warm day, once they’re stirred together, you have about 20 minutes of working time — depending on the amount you’re mixing. So the first warning is that you should add pigment to the resin and finalize the colour you need before you add any catalyst. That’s obvious, right? And so is this once Hewitt points it out: “Mix all your colour at once, then add hardener to the small amounts you need as you go.” That way, each layer you apply will be the same colour as the last one, and if you repair more than one spot, they will all at least match each other. Finally, be scientifically precise with the catalyst. Read the instructions. Too much hardener will change the final hue.

Once you have the colour more or less right, you can apply the gelcoat to the prepared area. Use a Popsicle stick or a plastic scraper or even a plasticized playing card to lay on as smooth a layer of the gel as possible. Keep in mind, of course, that “smooth” is an illusory ideal. Also keep in mind that you’re going to be sanding this whole area again after it sets anyway. Slightly overfill the hole to allow for shrinkage, and in cases where you need to fill a deeper depression, plan to build it up with two thinner coats, with a light sanding between. Now go away until tomorrow.

Putting on the polish

It bears noting that at this point the real job is complete. Gelcoat repairs are necessary in order to protect the fibreglass, and once you’ve done the prep and laid on the gelcoat, that end is achieved. From here on in, it’s all about aesthetics.

Carefully sand the area again using 120-, then 150-grit on a hardwood block to smooth out irregularities. Once you’re satisfied, Groen suggests you widen your work area slightly by moving the masking tape back a bit so that you see shine all around the patch area. You want the final coat to feather out onto the previously untouched surface, so use the 150-grit to rough up a narrow perimeter.

Apply a final layer — it shouldn’t reach all the way to your masking tape — and then, well, leave it overnight again. Next morning, you can begin sanding with 400-grit wet-dry sandpaper and progress to at least 600-grit or even 1,000-grit for highly visible spots. At that point, you can remove the masking tape and switch to rubbing compound buffed either manually or with an electric polisher. Note that, like sandpaper, rubbing compounds come in varying degrees of coarseness; for general duties like this, regular-grade should do the trick.

Finally, if you want to get really obsessive, apply a coating of wax. This is something that you might consider anyway since, on a newer hull, a good UV-blocking wax will preserve that deep gelcoat colour for a few years longer before it starts to oxidize and become chalky.

With luck, the finished patch will, more or less, match the hull. As Hewitt says, “I figure that anything you can’t see from three feet away is a success.”

This article was originally published on January 1, 2007


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Tom Carpenter