Polyurethane Glue vs Wood Glue

Discussion in 'Luthier's Corner' started by ScottTunes, Feb 7, 2013.


  1. ScottTunes

    ScottTunes

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    Basically, "Gorilla Glue" vs Titebond... Super Glue vs Elmer's...

    I need to fill a small gap between the upper body wing and the "neck-through" of a Rick v63, using glue through a syringe...

    I would prefer strength over fill qualities in this case... I think the poly glue expansion will fill better, but may not provide the holding strength of the yellow wood glue...

    Any opinions?
     
  2. MPU

    MPU

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    Epoxy would be my choise.
     
  3. Bruce Johnson

    Bruce Johnson Supporting Member

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    For that, I recommend Instrument Makers Glue, which is sold by LMI (Luthiers Mercantile). It's a variant on white glue. It can be squirted and forced down into tight spaces, and bonds very well. Unlike Titebond, it will fill gaps and it dries hard. It can be filed and sanded and won't sink under paint. It also dries clear, which is nice.

    Don't use polyurethane glues to fill gaps. That's like filling it with foam rubber. Polyurethane glues have their uses but this isn't one of them.

    The best, strongest glue for that job is a hard marine epoxy like West Systems. That's what I use for repairs of that type. But that's more complicated and expensive. I'm assuming that you don't want to go there.
     
  4. ScottTunes

    ScottTunes

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    Thanks for that!

    I've used West Sys long ago in wood boat repairs, and am familiar... I think they only sell large quantities, however (do they have quarts now?)... I will likely only need a tiny bit...

    I have Precision Shot poly and wood glue... I was leaning towards the wood glue...
     
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  6. Hopkins

    Hopkins Supporting Member

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    I would use thin CA glue, building it up slowly until the gap is filled.
     
  7. JoeDeF

    JoeDeF

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    They now do sell very small quantities. The smallest quantity is actually a pair of little foil envelopes (think of what mayonnaise comes in at a convenience store, only bigger) that you open and mix. The Handy Repair Pack contains two pairs of resin/hardener packs, filler, fiberglass tape, and mixing stuff. It would be more than sufficient, an would leave you with one unused resin/hardner pair for some other job in the future. Here's a link:

    http://www.amazon.com/West-Systems-HANDY-REPAIR-PACK/dp/B002IZDOCG

    If you don't want to pay for shipping, some local stores may carry it (where I am, the Rockler hardware store does, and I think some marine supply places might as well).
     
  8. ScottTunes

    ScottTunes

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    Thank you! I will definitely look into it this weekend!! Could be just the ticket!
     
  9. Bruce Johnson

    Bruce Johnson Supporting Member

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    A couple of tips about doing the repair with West Systems epoxy:

    Wipe some wax on the paint all around the repair area, on the front and back. I use Johnson's Paste Wax (no relation!). This will keep any squeeze-out from sticking. Be careful not to get any wax down into the gap.

    If the delamination gap goes all the way through the instrument (you can see daylight through it), then put masking tape on the back side and up around the edge. The epoxy will flow in pretty easily, and it will also pour right back out of the bottom. You need to contain it in there while it cures.

    West System epoxy needs to be mixed thoroughly, particularly in small quantities! Pour the two parts together into an old bottle cap or something and stir them together for at least a minute. A real minute, not 5 seconds.

    Set the bass up on blocks, so the back surface is up off the benchtop. It's really embarrassing to epoxy your bass down to the bench.....Lay a piece of waxed paper under it to catch any drips. If any drips form on the back side, it's better to let them harden as a drop hanging on the back. If you've waxed the paint, it's easy to knock off the hardened drop. It's harder to scrape off a hardened smear.

    Pour a little epoxy into the gap, wait a minute, pour in a little more, etc., etc. You can push it down in with a scrap of cardboard or plastic. But it will slowly sink down in there and fill up the gap over a period of about an hour. Keep checking back on it every ten minutes, adding another drop, until it's all full and won't take any more.

    Let it fully cure for about 12 hours before you start chipping off the drops and cleaning it up.

    By the way, the process is exactly the same if you are using the LMI Instrument Makers Glue. Except for the stirring.
     
  10. suraj

    suraj

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    Bruce's tips in any situation are so detailed, its almost like a walkthrough in real time..!! :D always look forward to your posts sir..
     
  11. Bruce Johnson

    Bruce Johnson Supporting Member

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    That detail comes from many years of experience. I've done gap fills and delamination repairs like that hundreds and hundreds of times, and I've tried all kinds of glues and techniques along the way. A steady part of my business here is restoring the old Ampeg Scroll Basses. They come to me in all kinds of horrible condition; cracked, delaminated, twisted, large mysterious holes, etc. I put them back together with West Systems epoxy. It's the strongest, most stable glue I've found for doing structural repair work. I also use it for almost all of the laminations and glue joints on my own line of basses. Plus I use it to cast my pickups.

    I'll mention again the LMI Instrument Makers Glue. It's an excellent product, an alternative to Titebond that's better suited for the work we do. I use it for many applications where I don't want to use epoxy, for various reasons. In particular, I use it for gluing fingerboards and headstock veneers on necks that I build for some clients. The epoxy will sometimes leave a darker line at the seam that customers may not want. The LMI glue is clear and nearly invisible. Plus, it's easier to take apart with heat, if needed. I also really like the LMI glue for gluing in frets. I lay a thin bead of it down in the slot with a syringe and press the fret in.
     
  12. ScottTunes

    ScottTunes

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    Thanks again, Bruce! Great information, much appreciated!

    There's a West Marine near me. Headed there Sunday or Monday...

    Sounds like LMI glue is better for the more "delicate" detail work of guitar building/repair than Titebond. How is it for cabinet making? Most of my wood projects are actually building speaker cabinets, and similar projects that require less skill, but still enjoyable for me.
     
  13. Bruce Johnson

    Bruce Johnson Supporting Member

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    Titebond is a classic woodworking glue, great for the majority of jobs in a wood shop. I keep bottles of Titebond I and II here in my shop, and I use them for many non-instrument projects.

    I stopped using Titebond on instruments about ten years ago, for several reasons. The main reason is its behavior under paint. My basses are all high-gloss relatively thin finishes, and Titebond will shrink and sink down in the joints. A few months to a year later, the seams will be big lines in the finish. Titebond II dries harder than the original Titebond I, but it still leaves some kind of lines under the finish. In comparison, the LMI glue dries hard and can be sanded smooth, and doesn't seem to shrink down. The West Systems epoxy doesn't shrink at all. I even use it as grain filler on woods like ash.

    Titebond also has trouble gluing hard dense woods like hard maple. I've had maple-to-maple laminations fail when glued with Titebond, as well as hard-to-softer combinations like hard maple to rosewood. I think the Titebond is formulated a little thick for woods like hard maple. It doesn't grab into the wood well enough unless you rough it up.

    Titebond is great for softer woods like birch plywood, Douglas Fir, and medium-weight hardwoods with more open grain. Structurally, it's fine for laminating body woods like alder, ash, and mahogany. But, you always have that issue with it sinking under the finish. Titebond is great for most cabinetmaking and furniture projects. The exceptions are highly stressed dense wood joints, and areas where sinking under a gloss finish will be noticeable. For those cases, use the LMI glue or the West Systems epoxy.
     
  14. ScottTunes

    ScottTunes

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    Wow! I'm now going to be adding LMI glue to my growing arsenal!
     

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