Which is the best way to epoxy glue but or scarf joints

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MageDK
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Which is the best way to epoxy glue but or scarf joints

Post by MageDK »

Morning all,

I am planning ahead to the first parts of my boat build (Trawler 28) and I was wondering what the best "Epoxy glue" to you. I know the HOW TO guides on the site mention creating epoxy glue using epoxy and wood flour. I have also seen on some videos and boat building sites that they mix colloidal silica with the epoxy to make epoxy glue used to join different pieces of wood (Either for but joints or scarf joints).

Which of the two is going to make a stronger epoxy glue?
The epoxy and wood flour
or
The epoxy and colloidal silica?
Check out my Trawler 28 build viewtopic.php?t=66261.

fallguy1000
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Re: Which is the best way to epoxy glue but or scarf joints

Post by fallguy1000 »

I don't know the answer.

Wood flour has an upside. It is not toxic. Prolonged exposure to fumed silica is bad as a known carcinogen.

Wood flour is hygroscopic, so any bits of wood flour that are exposed can take on water, but a good homogenous mix would not allow water to go very far.

I strictly use fumed silica, but breathing it is very bad. I like the way it flows in situ.

Butt joins require extra effort because end grains like to pull resins from the putty and then you have a weak putty. Precoat all end grains with clear epoxy and wait 30 minutes before putty bonding. For joints that will be tortured; fiberglass is required on both sides of the seam at 2" each side.

Scarf joints also need precoating, but done well, they do not require glass. I prefer a 1/16" or 1.5mm vee trowel on each side with the putty after the clear coat has kicked. I hate scarfing, for the record. So much work..
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MageDK
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Re: Which is the best way to epoxy glue but or scarf joints

Post by MageDK »

fallguy1000 wrote: Wed Jul 20, 2022 12:14 pm I don't know the answer.

Wood flour has an upside. It is not toxic. Prolonged exposure to fumed silica is bad as a known carcinogen.

Wood flour is hygroscopic, so any bits of wood flour that are exposed can take on water, but a good homogenous mix would not allow water to go very far.

I strictly use fumed silica, but breathing it is very bad. I like the way it flows in situ.

Butt joins require extra effort because end grains like to pull resins from the putty and then you have a weak putty. Precoat all end grains with clear epoxy and wait 30 minutes before putty bonding. For joints that will be tortured; fiberglass is required on both sides of the seam at 2" each side.

Scarf joints also need precoating, but done well, they do not require glass. I prefer a 1/16" or 1.5mm vee trowel on each side with the putty after the clear coat has kicked. I hate scarfing, for the record. So much work..
thanks for the heads up regarding the silica. I have a respirator I use for these types of projects, but no point is risking exposure unless the bonding strength is significantly higher.
Check out my Trawler 28 build viewtopic.php?t=66261.

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Re: Which is the best way to epoxy glue but or scarf joints

Post by cape man »

I have used nothing but wood flour for making thickened glue. Once thoroughly mixed with epoxy they are no longer hydroscopic, and best of all it is very inexpensive thickener. I also now use it exclusively for filets and fillers. It SUCKS as a fairing compound as it is very hard to sand!
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Re: Which is the best way to epoxy glue but or scarf joints

Post by piperdown »

cape man wrote: Wed Jul 20, 2022 1:39 pm I have used nothing but wood flour for making thickened glue. Once thoroughly mixed with epoxy they are no longer hydroscopic, and best of all it is very inexpensive thickener. I also now use it exclusively for filets and fillers. It SUCKS as a fairing compound as it is very hard to sand!
Same here, only wood flour. Cheap and makes a great bond.

As fallguy pointed out you need to precoat with neat epoxy first otherwise the wood will soak it up and make a dry joint. Get any clamps and bracing ready as even thickened the joint will want to slide around until it starts to set up.
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Re: Which is the best way to epoxy glue but or scarf joints

Post by wpstarling »

i also only use wood flour and find it very easy to work with. It's not hard to mix it well but I also try to pre-coat just about every surface before i put glue/putty/fabric/cloth/anything on it. I can't think of a situation where it hasn't made things easier. I will say that in my workshop which is a metal building I can get vastly different temperatures so my mixtures sometimes need to be a tad different. For example winter time I need just a bit less wood flour than the summer since the epoxy mixture is thicker.

Also as with anything you're going to add to the epoxy make sure you mix it before you add the wood flour or silica.

And I know you asked specifically about glue but for filets I find it easier to pour/scrape the thickened epoxy into a ziploc bag and cut the tip of it off and then I can really be sure the mixture gets in the joint and if I need to come back and smooth it or contour it in anyway it's pretty easy with a craft stick/tongue depressor.
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Re: Which is the best way to epoxy glue but or scarf joints

Post by BarraMan »

cape man wrote: Wed Jul 20, 2022 1:39 pm I have used nothing but wood flour for making thickened glue. Once thoroughly mixed with epoxy they are no longer hydroscopic, and best of all it is very inexpensive thickener. I also now use it exclusively for filets and fillers. It SUCKS as a fairing compound as it is very hard to sand!
Ditto! :lol:

MageDK
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Re: Which is the best way to epoxy glue but or scarf joints

Post by MageDK »

Thanks all for the additional suggestions about fillets, pre-coating, etc. I'm a first-time boat builder so more information is always good :)
Check out my Trawler 28 build viewtopic.php?t=66261.

joe2700
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Re: Which is the best way to epoxy glue but or scarf joints

Post by joe2700 »

Another option is premixed epoxy adhesive like system 3 gel magic, west system g flex, or total boat thixo. More expensive but with some better working and mechanical properties. Not at all required as mixing your own works fine, but the premixed stuff from whatever epoxy system you are using can be nice to work with.

If you mix your own I would use colloidal silica but yea don't breath the dust as you mix it in

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