Bonding to cured epoxy fibreglass

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OneWayTraffic
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Bonding to cured epoxy fibreglass

Post by OneWayTraffic » Tue Mar 10, 2020 3:05 am

As I have a major sanding project ahead of me with about 5 square metres of poorly laid glass to sand down to the good layer underneath I thought I would ask: How strong is the bond to sanded glass, where you absolutely have to cut into the glass itself. Would it be better to go down into wood, or is the secondary bond to glass still strong enough even if the sanding has removed all the sizing.



terrulian
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Re: Bonding to cured epoxy fibreglass

Post by terrulian » Tue Mar 10, 2020 7:06 am

If I understand your question, the difference is between a chemical and physical bond. Although the chemical bond, wet-on-wet, laying the next layer while the underlying layer is still curing, is preferable, it is often not possible. In those cases where the epoxy has cured, you merely sand for tooth and apply the epoxy/glass. Assuming you've sanded through all the dodgy stuff down to a solid surface, that's all that is necessary and is commonly done with no problems. After all, the bond to the wood is also a physical, not a chemical bond.
But perhaps I've misunderstood you?
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Re: Bonding to cured epoxy fibreglass

Post by fallguy1000 » Tue Mar 10, 2020 10:23 am

Avoid sanding deep into the wood. It will be tempting to dwell.

If your first layer of glass bonded well, just remove the top layer. Then clean it all off and look for non laminated areas.
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Re: Bonding to cured epoxy fibreglass

Post by fallguy1000 » Tue Mar 10, 2020 10:26 am

You will get good bonding over a 40 grit scratch. Yes, it is a secondary bond, but far better than the air pockets.

To remove everything to wood is not preferred, in my opinion.

If you have unevenness after sanding; you can spot fill and sand them level before glasswork.
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Re: Bonding to cured epoxy fibreglass

Post by fallguy1000 » Tue Mar 10, 2020 10:39 am

fyi One Way, the thing we do here is we have a 5 gallon bucket with about a half gallon of acetone in it and a lid.

When I am done with a first layer, I have a helper go dunk the roller.

Once in awhile we have had a non-solvent resistant handle go in there, but generally not.

After a day or hard work and sticky tools; it is really nice to throw them in a bucket and cap it off until the next day.
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Re: Bonding to cured epoxy fibreglass

Post by Reid » Tue Mar 10, 2020 12:08 pm

I agree with these guys. Sand down first layer of glass until you come to a layer that has a good lamination to the wood. You want to try and laminate your new glass to a nice even surface. If there are gaps/pockets or any ridges I would fair those first before laying the next layer of glass.
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Re: Bonding to cured epoxy fibreglass

Post by fallguy1000 » Tue Mar 10, 2020 1:41 pm

Another thing to note.

Dirt is an enemy of bonding. Once you are done, sand everything with a high powered shop vac or sweep it multiple times. Then clean with denatured alcohol.
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Re: Bonding to cured epoxy fibreglass

Post by OneWayTraffic » Tue Mar 10, 2020 2:03 pm

Thanks for the replies. What about places where I have to go to the wood? I intend to put a bigger radius on my transom to bottom joint for example. Ok to go back to wood a couple of inches away from the joint or still not really needed if the substrate is level, clean and fair?

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Re: Bonding to cured epoxy fibreglass

Post by fallguy1000 » Tue Mar 10, 2020 5:21 pm

OneWayTraffic wrote:
Tue Mar 10, 2020 2:03 pm
Thanks for the replies. What about places where I have to go to the wood? I intend to put a bigger radius on my transom to bottom joint for example. Ok to go back to wood a couple of inches away from the joint or still not really needed if the substrate is level, clean and fair?
Going to wood is fine. If your feathering looks sketchy, cut it off with a knife to good bond, then precoat the wood and those areas with epoxy and let it tack and then use thickened epoxy to fill low spots. Done same day, you can glass over it. Or u can glass within the bonding window, but usually I sand. You can also use peelply.

With your heat issues; best to fix the areas and resand before glassing.
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Re: Bonding to cured epoxy fibreglass

Post by OneWayTraffic » Tue Mar 10, 2020 5:49 pm

Yeah the heat issue is a factor. All my previous successful gluing and glassing was either done in a garage, or in the colder months. This time I'll wait until the worst of the days heat is over. Summer is done anyway. In a few months I'll be struggling with the cold. Hopefully that will be less of an issue, or I'll just wait it out for Spring.

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