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Thread: Back removal

  1. #1

    Default Back removal

    I've kinda finished my first mandolin - a slightly modified F5. Somehow, even with locator pins, the back is off center meaning more overhang on one side vs. the other. I guess I should say that these are violin edges thus the overhang.

    I glued the back on with fish glue. What would be the proper method for loosening the fish glue and removing the back. Does it matter where you start to release the back? How do I minimize damaging the back and rim?

    Thanks in advance - Gary Davis

  2. #2
    Registered User sunburst's Avatar
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    Default Re: Back removal

    I'm not experienced with fish glue, but from what I understand it is similar to hide glue when dried, although perhaps more hygroscopic. It may be very difficult to release the back without damage. How far off center is it? It might be best to live with it and move on rather than risk damage.

    As for the process of removal (if you go that route), it is similar for all glues: heat the joint and insert a thin blade into the glue line. Where to start? Usually wherever you can most easily insert the blade. Be particularly careful at the head block and tail block. Not only are those the most difficult parts to release simply because of the bigger glue surfaces, but the grain of the back is such that prying with the blade can easily split the back.
    Moisture can be your friend. Many people keep a vessel of hot water nearby and repeatedly dip the blade into it. You can use a steam needle once you have the joint open somewhere.
    Assuming you haven't applied any finish yet, more heat and moisture can be used because there is no danger or ruining the finish like when working in finished instruments.

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  4. #3
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    Default Re: Back removal

    Opening a mandolin is risky business.
    If you mean that the back is overhanging the sides, you might want to reconsider opening the instrument, and instead flush up the back to the sides.

    I have not used the stuff, but have read quite a bit about it.
    Conventional wisdom is that it is sensitive to a combination of heat and moisture.
    Some say it is too sensitive, which is why I have not used it.

    If no one with experience can give better guidance, you might test a section by applying a small amount of water to the seam, then let it sit, then come back with more water and a thin hot opening knife. Keep your knives hot and work very slowly. Once you have a spot open, it will be easier to inject a little more water into the seam. Work only perhaps 1/4" to 1/2" at a time.

    Start near a wide part of the sides, and work your way towards the waist. If you work the other way, you are more likely to crack the back.
    The blocks come last, and can be quite difficult to release. Try to get a little water in there, and work from more than one direction.

  5. #4
    Registered User amowry's Avatar
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    Default Re: Back removal

    With a violin you can usually remove the top/back dry, with a palette knife, because hide glue is brittle. I'm not sure how fish glue compares, though, and newer glue might be more difficult to remove than old glue. Violinmakers also sometimes recommend using alcohol instead of water when removing plates because it dries out the glue more, making it more brittle. The evaporating solvent also chills the glue joint. You'll find lots of info on the web about removing violin plates (including videos). Mandolins, not so much .

  6. #5

    Default Re: Back removal

    You have gotten excellent advice here already. I use a pot of boiling water to heat pallet knives. My question is: if there is overhang all the way around, why don't you just flush trim the back to the rims and put a small radius on the corner? Many builders do this in lieu of binding?

    Alternately, could you just trim the overhang to the minimum it is on one side, making it consistent all the way around?

  7. #6

    Default Re: Back removal

    Thanks y'all. It sounds like I need to do some soul searching to see if I can live with my results. I finished the edges and installed the purfling before I glued it on so I'm not sure what options I have.

    Here is an image of the top and neck before I attached the back.

    [IMG]Untitled by Gary Davis, on Flickr[/IMG]

  8. #7
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    Default Re: Back removal

    Alcohol might help, but test it first on areas that are not visible from the outside to make sure that the combination of alcohol and glue will not stain the wood.

    An alternative would be to flush up the edges, remove the purfling, re-cut the purfling channel, and install a different style of purfling.

    Did you use a mold?

    For future reference, in violin style construction the back is usually glued on first, then the top. The edges are trued up and the purfling channel cut only after the plates have been installed. Only then is the neck joint fitted and the neck glued on. Otherwise, it is likely that the geometry of the instrument will be faulty. The tailpiece comes last. For most other string instruments, the neck is still fitted and installed after the plates have been glued on.

    I would suggest you read a well accepted book on violin construction before you reassemble the instrument.
    Last edited by rcc56; Jun-13-2022 at 1:24pm.

  9. #8

    Default Re: Back removal

    I mentioned the back was glued on now - hence my query.

    I'm going to have to look at it again to see if your re-purfling option is viable with what I have together so far.

    It may just have to go to the burn pile and caulk it up to inexperience. Maybe I'll string it just to see what it sounds like then toss it. My son will not be happy.

    Thanks for your time - Gary

  10. #9
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    Default Re: Back removal

    Yes, I just saw that and have corrected my post. Sorry about that.

    Re-purfling may very well be an option. If you are creative enough with your choice of purfling, chances are good that you will have enough surface to make something work.

    And by all means, string it up and see how it sounds before you do anything else. If it sounds good, I'll bet that you can come up with a solution to your cosmetic problems.

    And don't throw it on the wood pile just yet. Instead, since the long term reliability of fish glue is still being debated, please leave it strung up for a few months even if it doesn't sound good, and let us know if the instrument holds together. You'll be helping the rest of us. We really need a lot more concrete information on the subject, and your instrument is a good test case.

    Another thing: I'd rather have an instrument with imperfections that sounds good than a beautiful looking instrument that sounds bad. They're made to be played . . .
    Last edited by rcc56; Jun-13-2022 at 11:50am.

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    Default Re: Back removal

    There is another option to remove the back with minimal damage. Get a thin NO kerf saw and saw the glue line. Violins use diluted glue for the top and back so they are easier to remove. I have sawed the back off guitars, just go slow and follow line. Thin saw with no kerf is the key. I have read Randy Woods saws off backs to repair mandolins. With no kerf you will lose a minimal amount of material from the sides, less than a half mm. Good luck.
    Last edited by pops1; Jun-13-2022 at 1:26pm.
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  13. #11
    Registered User sunburst's Avatar
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    Default Re: Back removal

    The thinnest saw that I have has a .008" kerf. I've always wished for a saw with no kerf, but alas it does not exist.

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    Default Re: Back removal

    Fish glue releases very easy with a small amount of steam.

    I use one of the $20 hand held steamers off Epay. If you don't have one of those, a simple old school coffee pot with a 50 cent hose coming out of the "whistle hole" or a $3 thrift store espresso machine work great.

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    Default Re: Back removal

    Quote Originally Posted by sunburst View Post
    The thinnest saw that I have has a .008" kerf. I've always wished for a saw with no kerf, but alas it does not exist.
    I measured mine, it is .01 thickness for the blade and it does seem to have some kerf as it measures .012 at the teeth. It was sold as a no kerf saw decades ago and came from England. I don't remember where I ordered it from. It is hard to use as it binds easily. It did saw the back off a guitar quite well tho.
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  17. #14

    Default Re: Back removal

    To offer another thought, it does not look horrible for a first build.

    I have built three and am working on #4 and 5, all A models. All of the first three have flaws. I played them anyway. Each one was a learning experience and an improvement. Number three actually sounds and plays decent despite worse cosmetic flaws than yours has. There is a lot to be learned just by playing it for a while. As Bob said,seeing whether the fish glue survives structurally would be new information, even to an extremely experienced luthier like him. It may come apart in a month or three. Most experienced builders seem to be saying the first five or ten instruments are a steep learning curve.

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    Default Re: Back removal

    Fish glue will separate nicely with a heated palette knife. (I think it's great stuff and use it a lot.) Depending on how much finish you have on the instrument, you might be able to soften the joint with a heat gun prior to separating it.

    As CarlM said, you could just suck it up and accept the flaws as part of the learning process. It'll likely go better next time.

    My first instrument hangs on the wall in my office as a reminder. As I've told people, "That's Number One, but it looks like Number Two."

    I also have a Wall of Shame in the shop, where a couple of very humiliating mistakes hang. In a humorous and unintended coincidence, the Wall of Shame examples hang right next to a fire extinguisher and a first aid kit.

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  20. #16

    Default Re: Back removal

    OK - I'll string it up as is and see what happens. I'll try to remember to report back here in a couple of months or so. Then I may or may not proceed with steaming off the back.

    Thanks y'all - Gary

  21. #17

    Default Re: Back removal

    I think it looks very nice, Gary.

  22. #18

    Default Re: Back removal

    I think it looks good from the front. About the back...you could say that you removed the back to get the Virzi out...

  23. #19
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    Default Re: Back removal

    Yep, it looks just fine from the front. Put a ToneGard on it and nobody will ever know.

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  25. #20
    Fingertips of leather Bill McCall's Avatar
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    Default Re: Back removal

    And if it sounds good, nobody will ever care.
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  27. #21

    Default Re: Back removal

    Thanks for the kind words y'all.

    Gary

  28. #22

    Default Re: Back removal

    Quote Originally Posted by Jonathan Ward View Post
    Yep, it looks just fine from the front. Put a ToneGard on it and nobody will ever know.
    Thanks for that idea. I checked them out. It might just be enough to distort reality.

  29. #23

    Default Re: Back removal

    I was over commiserating about my situation and thought I might sand the cherry back a bit just for fun. Its looking nice and tugging at me to finish her.

    My purfling channels are far from perfect. As I was sanding I got to thinking how I might be able to use all this sawdust to kinda fill in my imperfect purfling channels.

    Anyone care to share some wisdom about such an endeavor?

    Gotta drive to Casper today to pick up a cello. Be back later.

    Thanks so much - Gary

  30. #24
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    Default Re: Back removal

    Expect the cherry to darken when you fill with the dust. Except for ebony, I use a lighter wood dust to fill so when it darkens it looks more like the wood I am trying to fill. Example, I use mahogany dust for filling rosewood, unless the rosewood is very dark. I am usually doing fingerboards tho and not a top or back.
    Last edited by pops1; Jun-16-2022 at 11:29am.
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  31. #25
    Registered User sunburst's Avatar
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    Default Re: Back removal

    Filling with nearly anything will look better than wood dust. You can get a similar look using plain ol' hardware store wood filler with much less work. You can't hide filler in the purfling channels unless you 'paint over it' with a very dark stain so something contrasting might be better.

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