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Discussion Starter #1
This is an unsightly crack on a boat I help out with. As you can see someone smeared some gunk on it and made it worse.
What would you recommend for a repair?
I'm showing a closeup and a distance so you can see where it is on the boat.
 

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This kind of crack is common, and can be hard to repair well. Many people just leave them.

The key to a good near-invisible repair is to find a gelcoat to match the rest of the hull.

In a similar situation I used a cabinet scraper (i.e. sharp metal edge) to widen and clean out the cracks, cleaned thoroughly with acetone, then filled the crack with gelcoat. When dry, it could then be sanded flat and polished.

Faint while lines were visible after the job, but to me that was better than the cracks.
 

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Discussion Starter #3
I'm worried about what caused the crack. Is their some weakness that has to be fixed so it will not just crack again.
 

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I'm worried about what caused the crack. Is their some weakness that has to be fixed so it will not just crack again.
It looks like a case of spider-web cracking. It's when the area is loaded and the gelcoat, which is not as flexible, can't flex as much as the fibreglass underneath, and cracks. You'll probably find that the underlying fibreglass is OK.

It happens in places that are regularly jumped on, like cockpits. I bet that spot gets stepped on every time you get on and off the boat and even jumped on when you come down from working on deck.

If it was me, I would do as the previous reply-er suggested, but with the addition of drilling the ends of that biggest crack to stop it propagating.
 

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Use a rotary tool with a conical grinder tip to dig it out. Don't be too alarmed if small bits chip off, just fair them in with the grinder. Then do the gelcoat/sand/polish thing.
 

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There must be some flexing in that area leading to the crack. It'll be hard to stop the crack from returning without first stopping the movement that created the cracks.
Stopping that movement may not be so easy...
 

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Easy way is toplace slight upwards pressure on the area to open up the cracks, then clean them out well with acetone etc, Then using thinned gel coat, or thinned paint, fill the cracks up. With paint, do it in several applications as each 'coat' will shrink a bit.
when they're full ease off on the pressure, but don't completely remove it (scissor type car jack works well for that purpose) so they close back up. when it sets up, bond and then glass a plywood backer under the area to spread the load.
I normally use paint because it's more flexible than gel coat and seems to last longer.
I've done those repairs several times with both materials, but have only ever had the cracks come back with gel coat
 
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