Crack repair - deep

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Crack repair - deep

Postby fatjackdurham » Wed Jul 26, 2017 7:50 pm

I have a crack in the cockpit that seems to go all the way through judging by the slight flex of the fiberglass on one side but not the other.

What is the best procedure for repairing this kind of crack? The only thing I could think of was to put a swatch of fiberglass fabric over it and epoxy it, but that will make a pretty ugly sight. If I grind out the crack and fill it with epoxy, I am afraid it won't bond strong enough. What's the best thing to do here? And also, what can be done about hairline cracks in gel coat?

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I saw this video on youtube. Is this technique sufficient if I have an actual air gap after grinding out the crack?

https://www.youtube.com/watch?v=339W-ZNC1_0
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Re: Crack repair - deep

Postby GreenLake » Wed Jul 26, 2017 9:15 pm

There are gel-coat cracks and then there are cracks in the laminate. If you can push one side, it seems you have a crack all the way through the laminate.

My approach would be to grind the area around the crack to a 1:12 bevel and then fill back in with fiberglass cloth (!) and epoxy. Epoxy gives you a stronger bond which is something to think about when you suspect that the original laminate wasn't strong enough.

Because you can't really get at the area from below, your only chance at reinforcing it is by adding a layer on the top. That will be visible as a raised area, but you could try to otherwise make it look like a neat patch (square edges, sanded smooth, etc.) Thing is, with epoxy, you need to cover it against UV.

Normally, people say paint it, but if you use a no-blush epoxy (like System Three Silvertip) and mix it in very accurate proportions then coating it with gelcoat like for the repair shown in the video, should be possible. You could also get a more specialized product, such as SB112 from SystemThree which is actually formulated for use with gelcoat (intended for building surf boards).

You could of course use polyester resin as shown; it may be strong enough, after all. (Can't really tell where in your boat that crack is located). Worst will happen it cracks again.

If you grind a bit too far, I wouldn't sweat a gap if it's 1/2" or less If you fill your crack by putting the largest patch down first and then succeedingly smaller patches the first patch of cloth will be well anchored on the sides of your repair. It will dip through a bit, but I assume you don't care that the underside is smooth. If you worked by putting the narrowest one down first, that could push through.
~ green ~ lake ~ ~
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Re: Crack repair - deep

Postby fatjackdurham » Thu Jul 27, 2017 9:03 am

Thanks, GreenLake.

I am curious, based on the video, laminating resins won't cure if exposed to the air. Does that mean, if I grind out the crack and there is a gap, that the bottom side of the patch will never cure?

I wonder if I should first lay a thin patch with curable resin, let it cure, then fill in the rest as normal with laminating resin.
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Re: Crack repair - deep

Postby GreenLake » Thu Jul 27, 2017 6:36 pm

You avoid that situation by using epoxy resins. Their curing isn't inhibited by air.

The stuff that requires air to be excluded is fine for people doing lay-up in molds, but really too much of a pain to work with for ordinary repair work.

That said, many years ago, I used glass and polyester resin for autobody repair and that stuff cured in air... who knows whether it would deal with being below the waterline on a boat - some autobody products such as bondo, are said to not do well with permanent moisture.

Interestingly, you can get gelcoat that cures in air, but at least where I shop it's stocked only in clear (not white). I've used it to cover exposed laminate in the bilge (where something had rubbed through). Worked fine for the purpose.

One material I've used that's polyester based and cures in air is 3M High Strength Marine filler. It's a paste that is reinforced with very short bits of fiberglass in it. I've used it to repair cracks and fill nicks. It will bridge up to 3/8" or so, and is strong enough to stand proud (e.g. on the leading edge of a center board or rudder). It's probably not strong enough to support your crack by itself, especially if someone can stand on or push against the laminate on one side of the crack.

But it might be a reasonable choice to start off a repair, but only if you build it up beyond the original surface by a reinforcing patch.
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