Removing glued parts and gluing new parts. Precautions regarding already penetrated glue?

Discussion in 'Wood Construction' started by Orsovolante, Dec 10, 2019.

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  1. Dec 10, 2019 #1

    Orsovolante

    Orsovolante

    Orsovolante

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    In the repair of the cell of the FP404, I have removed numerous parts of wood separating them along the gluing line.
    Now I have to paste the new pieces and a doubt arises. How much do I have to worry about the previous glue, penetrated where I'm going to glue the new piece? Clearly I prepare the surface, use abrasive paper etc ... but can I trust that the epoxy will bind sufficiently with that already absorbed by the parts of wood that I have not removed?
     
  2. Dec 10, 2019 #2

    wsimpso1

    wsimpso1

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    Best to get back to wood or composite, and then, immediately before bonding, sand the faying surfaces with 200 grit and do a solvent then dry wipe. This energizes the surface of cured epoxy for best bond. I mean immediately, if you wait an hour, you gotta do it again...

    If the "glue" is something other than epoxy, or I could not be sure it was epoxy, I would sand it all out.

    If reaching is tough, well there are rotary tools and detail sanders at Home Depot and little air powered belt sanders and then you can make sanding sticks to suit.

    Billski
     
    Last edited: Dec 10, 2019
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  3. Dec 10, 2019 #3

    Rockiedog2

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    I've done a lot of that and had same concerns as you. Talked to T88 about it about 40 years ago and they said it;s ok go ahead. I take care with it and still don't like doing it. Like you said, prep is important. I use coarse paper and tools for the mechanical bond, which is obviously the only kind you got. Get all the dust out.
     
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  4. Dec 10, 2019 #4

    TFF

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    Epoxy does ok bonding to epoxy. Other glues to themselves, not so much. If all you did was cut the glue line and sanded but never sanded into the wood, I would say you need to go deeper. If you have sanded back and a lot of the epoxy is gone, it will be ok. Your repair is more critical than lots of them, more by the book than less is to your favor.
     
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  5. Dec 10, 2019 #5

    Orsovolante

    Orsovolante

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    I used one of those oscillating tools on which you can mount different types of accessories.
    A pointed spatula with an abrasive side allowed me to work fairly accurately.
    In the past I have had the opportunity to make repairs on composite vehicles, in particular sailplanes, but generally the surface can be greatly enlarged simply by increasing the size of the fabric pieces. for which I had not posed particular problems for the resilience of the resin, on another already hardened.
     
  6. Dec 10, 2019 #6

    wsimpso1

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    You understand the topic...

    Not your first rodeo...

    As noted above, many glues do not bond to each other at all well, so anything but epoxy has to come out, and you are doing that.

    If you can get to wood everywhere, the bond is a new bond.

    One thing I would want to check is if this design has any history of inflight breakups initiating at wing mounts or of finding crushed wood in these areas during inspections. If so, maybe the wreckage should have been left to lie...

    Epoxy on epoxy and epoxy on fiber-epoxy with old sanding (anything over an hour is old) is all mechanical (as noted above), but if you do sand - solvent then dry wipe - apply epoxy in short order, you mess up bonds in the surface and can get a big fraction of a full fresh join strength. Not making this up, this is how the aerospace contractors specify repairs in their composite stuff.

    All this being said, let's remember what is going on. The epoxy connecting the new spars to the old fuselage and new pieces in between spreads the loads out in both parts and keeps them from sliding around. Spreading the loads avoids point loads that break fibers, preventing sliding prevents scraping away fibers. With replacing any pieces that look damaged, good prep, decently small gaps, and enough resin to fill gaps, you will be reproducing almost all of the original joints. The tiny bit you can not reproduce, well, most of us believe that these sorts of joints are overbuilt for carrying these loads, so you will usually be OK.

    Billski
     
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