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Thread: Bowlback repair of missing back strip

  1. #1
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    Default Bowlback repair of missing back strip

    So I acquired this one many years ago with the intent to scavange parts and its case.
    But it just sat in my closet. Since covid I have been resurrecting many projects and this is one of them, although its now a resto rather than a scavange.

    So my question is how to best replace the missing strip.
    I have sourced some birdseye maple veneer (like original) and made a clamping cawl of the approx shape. My plan is to cut several veneer strips and laminate them in the shape of the cawl using bottle hide glue between layers. Then shave and sand it into the correct width to fit the missing piece and finally glue it in with bottle hide. Thinking to match the crack line of original piece rather than to remove the whole piece. Would this make sense? What advice might you provide to me in this repair? I am novice in instrument repair but dabble and am soemwhat handy. Don't have many special tools.

    Also any thougts about matching the color and finishing? I like perfect although know that is likely not realistic...

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  2. #2
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    Default Re: Bowlback repair of missing back strip

    It's called a "stave" and you're missing a piece of it. That will be a labor of love and there really isn't a wrong way to do it because I don't know anybody that would do it. Best of luck, keep us in the loop and I'm not really sure matching the color will be your biggest problem. If you can fill that gap and it looks nice and smooth that should be impressive enough.
    "Bargain instruments are no bargains if you can't play them". These are the words of J. Garber.

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  4. #3
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    Default Re: Bowlback repair of missing back strip

    It would probably be easier to replace the whole stave than to try to make the new piece fit the irregular line of the break.

    If you use bottled hide glue for the stave, make sure it is fresh. The stuff should be discarded 12 months after the date of manufacture. Go to the Titebond website for information on how to read the date code.

    Hot hide glue would be better if you can get it clamped in place within 90 seconds or so.
    Avoid fish glue.

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  6. #4

    Default Re: Bowlback repair of missing back strip

    Out of the closet and on to the bench - good deal.
    Those backs scare me.
    Does it hold up to string tension now?

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  8. #5

    Default Re: Bowlback repair of missing back strip

    Nice project for these doldrums. If you succeeded in an invisible repair including that crack, it would be something to brag about for several reasons, so the much more practical thing is to slice out the broken part, or melt the glue. The good news is that each stave has it’s own coloration and graining, so the new one doesn’t have to match anything but approximate color. The other good news is that this isn’t a structural or acoustic issue, I think, so thickness and strength of join don’t matter. If it were my project, I might even use only one thickness of veneer (you didn’t say how thick the stock you have is), because glue and fabric is what’s going to provide strength on the inside. One thick layer would be easier because, to get the seams to match the other staves, I think you’d want to glue it in a little proud and shave it down rather than try for flush, and you’d not want to sand or scrape through one thin veneer into another, which would look nasty.
    Glue and clamping are obviously difficult if the top is still on the instrument, so there’s no interior form or caul. One way around this might be welders panel clamps which operate from one side and are removable, or something similar that you could make. Don’t know if there’s an equivalent in lutherie. Then reinforce the inside with fabric and glue as original, and shave down the outside and apply finish. Easy, no?

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    Default Re: Bowlback repair of missing back strip

    The veneer is 1/42" which I think is pretty standard.
    The original stave is about 2mm or .079"
    A three veneer laminate would be .071" which should be close.

    If I do, how best to remove the original broken stave?

    Haven't yet tried string tension. No reason to doubt it.
    But what strings do you recommend for bowlback?
    Thinking D-Addarrio light (10-38) Phospher Bronze
    Also need a nut. Thinking of Tusq XL (lots of success with this brand on guitars).
    I don't have nut files so cutting a bone blank is just more tools needed

    Pic below of the current progess to date with veneers, paper cutout and laminate cawl.

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    Any advise on how to edit original? New to forum and can't find an edit button...

  11. #7
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    Default Re: Bowlback repair of missing back strip

    . . . How to remove the original stave?
    Use a very sharp knife, and clean up the work with a small sanding block or file.

    String gauge?
    Lighter is better. Perhaps 9 to 34 on a bowlback.

    Pre-cut nut?
    The string spacing would probably not be right for the mandolin. And any pre-cut slot depths would still have to be adjusted for the instrument to play well.

    Stringing the instrument up to pitch with the missing staff?
    I wouldn't.

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    Default Re: Bowlback repair of missing back strip

    Quote Originally Posted by rcc56 View Post
    . . . How to remove the original stave?
    Use a very sharp knife, and clean up the work with a small sanding block or file.
    Do I use heat? and or water to soften the glue?

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    Default Re: Bowlback repair of missing back strip

    Quote Originally Posted by scstill View Post
    Do I use heat? and or water to soften the glue?
    You could carve it out to the glue line and then clean the glue off
    THE WORLD IS A BETTER PLACE JUST FOR YOUR SMILE!

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  16. #10

    Default Re: Bowlback repair of missing back strip

    I've been playing a bit with bowl backs recently so here's my 2 cents worth of advice.... I'd agree that the best way to proceed is to remove the entire stave and replace replace it. This also allows you to use what you take out as a template for your new section. I'd use a combination of heat and moisture to loosen the stave. This would have been made with hide glue and gentle heat and moisture will soften the glue enough to work the piece out slowly. A damp paper towel set over just the appropriate seams for a while followed by a hairdryer could do the trick, though it might take a couple cycles of that. Be careful not to burn the finish. The trick would be to avoid loosening the next staves over.

    No reason this can't turn out well with a bit of patience. I don't know, but I bet the stave edges are angled a bit to create the bowl. Copy that off the piece you remove. Good luck!

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  18. #11

    Default Re: Bowlback repair of missing back strip

    While you’re at it, and have that area open, it would be a good idea to have a careful look at the top, braces and all the joints from inside. Loose parts can sometimes be found by tapping on the outside.
    Inspired by this thread, and my new antique on the bench, I made a small run of a version of the welder’s panel clamp, using a thinner blade (.020”). Figuring out how to make one especially suitable for instruments resulted in “improvements” but I won’t know how useful until I get to that stage in the repair.

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    Default Re: Bowlback repair of missing back strip

    Here are a few pictures of the continuing journey.
    Two staves (one for backup) made from double layer birdseye veneer glued dry and clamped between to thin plexi sheets to forming caul.
    When removed I used the forming caul as cutting surface and paper template for pattern to cut a rough shape with exacto and then sanded the sides down to match the opening (slightly larger).
    They are very close in shape, thickness and curvature.

    Next step is to remove the old stave and final fitting.
    I have been trying to match the color on scraps but its really tough - any thoughts?
    Thinking that I should use an old inner tube to wrap around the entire mandolin as clamp - do you think its needed and correct approach?

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    Last edited by scstill; Jun-04-2020 at 3:04pm.

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    Default Re: Bowlback repair of missing back strip

    The broken stave is out.
    I taped the cut line and carefully followed it with exacto. Used a little water along the line.
    The heel block was a little challenging with glue on the bottom of stave.

    I have started to final fit the new stave
    I think I will have to glue it in sections because it needs special coaxing along its length

    I need an idea for supporting the stave from the inside while being glued.
    Any good ideas?

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  22. #14
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    Default Re: Bowlback repair of missing back strip

    You're going to have to get creative.

    You could make some cleats and glue them to the adjacent staves before you glue the new stave.
    Or you could install a thin lip to the inside of the adjacent staves to provide a good gluing surface. You could use some of your veneer, or some spruce worked thin.
    One or the other would be wise on a repair of this nature.

    Someone might suggest mitering the new stave in, but that would probably be really hard to do.

    And/or you can use thin hardwood dowels as jacks, possibly with a larger piece of material stuck on the end to increase the bearing surface, and knock them out or cut them out when the gluing is complete. Be sure to tape off any part of the jack that might come in contact with the glue, or it will be in there for good. Or use styrofoam strips as jacks.

    And if you're going to use the Titebond liquid hide glue, make sure the bottle is no more than a year old. Newer would be better. Titebond date codes can be found on their website.

    Examine the old stave carefully. Is the color in the wood, or in the varnish? Is the old finish soluble in alcohol? If so, it is either a French polished shellac, possibly with some sandarac added; or a brushed on spirit varnish similar to what was often being used on violins from the same period. If it is not water soluble, it is probably a violin oil varnish.

    You could try aniline dyes, either as a stain or in the varnish. I think I see a light tint of pure brown [mostly sold as dark brown or golden brown from the violin supply houses] and maybe some amber.

    Make up a test board and try different things.

    Good luck. If you get tight glue joints, no significant distortion, and a passable finish match, you'll have done better than most folks can do.
    Last edited by rcc56; Jun-04-2020 at 11:57pm.

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    Default Re: Bowlback repair of missing back strip

    Still trying to match the stain and finish - not very easy. Going to go to HD and see if I can get a couple of stains.
    The original stave has alot of color tones, brown, red, even some yellow. Its maddening. I get something close then take it outside and its way off.
    Wiping poly gloss (minwax) has been suggested as something that can be layered on and possibly match the finish.

    I tried to make some dowel jacks with a flexible plexi on the end. seems to be a fail for me trying to get these properly located.
    Thinking to go the way of glued cleats or strip.

    The inside is black, either painted to glued fabric. Thinking to just go with a flat black. Lowest concern but any thoughts?

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    Default Re: Bowlback repair of missing back strip

    I don't think you'll get a match with a poly product. You can give it a try on your test board, though. I might be wrong.
    But I believe that the chances of getting a match with modern Home Depot products are small.

    Your best chances of getting reasonably close on the finish is to use the same kind of products that were used when the instrument was built. Go to International Violin Company or Metropolitan Music. They specialize in violin supplies, and this instrument was probably finished with varnishes that were common in violin manufacturing around the turn of last century. These suppliers carry the closest things we've got to the stuff used in the old days.

    Get some shellac flakes, powdered aniline dyes, and maybe a small bottle of violin makers' oil varnish. Dissolve your shellac flakes and aniline dyes in pure grain alcohol from the liquor store. Avoid using denatured alcohol-- it can cause color changes and drying problems.

    Matching finishes is very difficult. The best we can settle for is sometimes 3 or 4 shades away from a match.

    I recommend that you take care of the carpentry first. You might have to experiment for weeks with test boards before you get a passable finish formula.

  26. #17
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    Default Re: Bowlback repair of missing back strip

    It would have built originally over a form (similar to those below). That kind of goes out the window if you don't remove the top. You could try some flexible strips of veneer running horizontal to the stave and held in place by some sort of inflatable bladder. You just need to be careful not to explode the instrument with the bladder. Think Inner tube. that would take care of the inside if it worked. I can't guarantee it will. You kind of need to think outside the box here.

    The cloth lining was never structural, some builders used paper, some used nothing at all. If it had a purpose other than cosmetic it might have been used to allow the bowl to not stick to the form.
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    "Bargain instruments are no bargains if you can't play them". These are the words of J. Garber.

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    Default Re: Bowlback repair of missing back strip

    The carpentry is pretty much done. The strip will fit really well after I get a way to support it.

    I have been thinking to stain and finish the strip before I glue in place. Is that what you recommend?

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    Default Re: Bowlback repair of missing back strip

    I suppose you could. If you do, the finish must cure very well before glueing, and any finish that gets on the glueing surfaces must be removed before the piece is installed.

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    Default Re: Bowlback repair of missing back strip

    Another dilemma....

    I stopped humidifying the mandolin for a few days, allowing it to sit in the room with windows open (dry here in SoCal)
    The opening is now slightly wider than when I cut the replacement stave.
    With slight presure I can bring the edges together. But I do not know if this the right way to assemble.
    Seems like a little pressure might be normal but don't need it to crack open after all this work
    so..... Should I humidify then glue, should I dehumidify then glue with pressure, or should I recut the stave to dehumidified measurements?

    If you were finishing this would you assemble with bare stave then tape off and finish?
    or would you finished the loose stave, sand edges, then assemble?

  30. #21
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    Default Re: Bowlback repair of missing back strip

    I would let it sit for a few more days until you feel confident that the instrument has reached equilibrium. Slight pressure either way should be acceptable. Heavy pressure would not be acceptable.

    Whether to finish before or after assembly is a judgement call. Each course has its own set of difficulties. If you finish after assembly, you will have to find a way to blend the new finish with the surrounding areas, and you might get a dark line at the joints. If you pre-finish, you risk damaging the new finish upon assembly, and you will have make sure the joints are completely clear of finish before assembly.

    As I think about this one, I might choose to pre-finish; but if I did, I might wait several weeks to let the finish cure before assembly.

    You have chosen a difficult and unusual repair for your first major job. You are going to have to use your common sense to guide you. Most of us have not replaced a rib on a bowl-back instrument. Sometimes, the work itself reveals the best way to proceed as the job progresses.
    Last edited by rcc56; Jun-10-2020 at 12:04pm.

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  32. #22
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    Default Re: Bowlback repair of missing back strip

    So I decided to reglue the "internal strap" that had loosened from the adjacent staves as a way to pull the bowl together.
    I hope I didn't create a monster by doing this.
    Some additional separation has occurred at one of the staves as well as at the "wrap around wood" at the tail
    I think I can work glue into the separations and clamp.
    Thoughts on the strap glue? It succeeded by closing the opening about 3/32
    I currently having it in a humdifying bag
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  33. #23
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    Default Re: Bowlback repair of missing back strip

    It's time to stop "bagging" the instrument and let it reach a point of natural equilibrium. Only after it has sat at equilibrium for a while will you be able to close things up without something else opening up later.

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    Default Re: Bowlback repair of missing back strip

    I would glue internal ledges of some sort on the inside for the new stave to rest on. And this is also a great place to use some rare earth magnets on the inside and out as clamps. I'd glue the new piece in in stages.
    As to finishing, do you have an airbrush or something like that? Then you could mask off the adjacent pieces to color the new one. And I'd use dye which you're not going to find at HD.

  35. #25
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    Default Re: Bowlback repair of missing back strip

    And if you use magnets, mask the magnets with mylar tape so you don't unintentionally glue them to the work.

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