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Thread: Sprayed Glue?

  1. #1
    Luthier Tom Haywood's Avatar
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    Default Sprayed Glue?

    I'm looking at the inside of a cheaper Pac Rim mandolin that appears to be put together with sprayed on glue. The dried glue is extremely thin, full of air bubbles, and is non-existent in many places. I don't see any indication of it being brushed or wiped on. It is very brittle between the connected pieces. The loose residue is extremely thin paper-like material.

    Is it possible that some kind of glue is being sprayed?

  2. #2

    Default Re: Sprayed Glue?

    The only thing I can think of is aerosolized glue, like the kind that 3M makes. I keep a can in the shop for making templates--spray on plywood or acrylic, stick the paper template on and flatten. The cut it out. The times that I have over-sprayed that product the resulting dried bits that get on my scraps look like you describe. But I don't know why anyone would adhere any part of an instrument with that stuff.

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  4. #3

    Default Re: Sprayed Glue?

    Any glue can be sprayed; even HHG. The plywood folks probably started that long ago; the auto industry uses robotic structural glue applications, so I can’t see any reason a high-volume instrument maker wouldn’t.
    But it begs the question of why wood instruments aren’t finished on the inside, since sealing both sides of a slab is the traditional way to avoid humidity related warping, and veneers, especially have to have both sides balanced.
    That said; I’m excited today because I’m getting another condition unknown Oscar Schmidt 12 string bowlback. Curious to see if it’s as surprisingly good as the other one I have: lots of bass from a tiny round box. Also, how these things can hold together for a century!
    I know it needs a few inches of that black/white segmented binding that I have to make, and at least one top crack closed, plus a little repair to one tuner. Anything to get away from the news.

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  6. #4
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    Default Re: Sprayed Glue?

    "Is it possible that some kind of glue is being sprayed?"

    Yes, it's possible. If so, I do not know what the expected production advantages would be. If the manufacturer has a rash of glue joint failures, it will cost them more than any labor time they might have saved by spraying.

    To me, it seems like just another bad idea, like Gibson's experiment with infra-red cured glue in the '70's.
    I could add to the list of so-called cost saving factory techniques that were of dubious value ad infinitum, but I'll stop here.

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  8. #5
    Luthier Tom Haywood's Avatar
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    Default Re: Sprayed Glue?

    Here is the best photo I can get showing the glue residue, the lack of glue, and the failure at the rib/neck block. This is a popular brand, so this could show up again. This mandolin is about 5 years old. It really felt like the finish was the only thing holding it together. I have seen a photo of another one of these that appears to have the same neck block failure.

    Click image for larger version. 

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  9. #6

    Default Re: Sprayed Glue?

    That looks like a poorly-fit joint or gap, and good old fashioned PVA. Maybe thinned a bit and brushed on. Or maybe masked with a template and brushed on, hence the clean-ish-looking ring of glue.

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  11. #7
    Registered User sunburst's Avatar
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    Default Re: Sprayed Glue?

    It is possible that they spray glue, but I know that some manufacturers have things kind of like blotters or sponges in the shape of the part to be glued. The worker would simply press the part down against the glue-soaked blotter and slap it together.
    Last edited by sunburst; Jun-01-2020 at 11:02pm. Reason: spelling

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  13. #8
    Luthier Tom Haywood's Avatar
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    Default Re: Sprayed Glue?

    John, I hadn't thought about blotting. That makes more sense and explains what I'm looking at. Thank you.

    Marty, the residue does look a bit like thinned PVA, but it looks more like a thick clear CA. What really had me confused is that everything came apart as if sticky note glue was holding it together. The rib was properly fit to the neck block, but whatever glue was in there no longer exists. The result was that the block rotated on one side under pressure, making it appear that the neck joint had failed. Thanks. I hope y'all are staying safe with all this stuff going on around town.

  14. #9
    Adrian Minarovic
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    Default Re: Sprayed Glue?

    Quote Originally Posted by sunburst View Post
    It is possible that they spray glue, but I know that some manufacturers have things kind of like blotters or sponges in the shape of the part to be glued. The worker would simply press the part down against the glue-soaked blotter and slap it together.
    That's it. I've seen some vids and pics from factories that use a roller half submerged in a pan of glue. They just roll the glued edge over the roller and clamp. The glue may be a bit diluted to work this way (or they add water as the glue in open pan thickens during day)
    Adrian

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  16. #10
    Luthier Tom Haywood's Avatar
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    Default Re: Sprayed Glue?

    Mystery solved, I think. This appears to be some kind of hide glue. Rubbing with warm water removed all the glue residue, which wasn't much. A little sanding some of the residue brought out the smell of hide glue. It may have been blotted on thin and fairly evenly, but it shrank and left critical areas with no glue. I guess if it holds together past the warranty, the seller is happy.

  17. #11
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    Default Re: Sprayed Glue?

    Could it possibly have been fish glue???
    Or maybe it was Gibson's famous '40's - '50's glue where "the foreman started the morning by sprinkling a little glue into the water pot"-- as a fellow luthier friend of mine once said.

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