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viola_license_revoked

Warped plywood top, how to fix?

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hello all

I'm fixing a broken bass for a kid who cannot afford a real luthier. A sizzling plate of arroz con pollo is at stake here! Any suggestions please?

The bass bar had separated from the top about 2/3 of it's length. The kid played on it like that since he got it donated for all of the last 2-3 years in youth symphony.

What can I try to correct the sunken/warped plywood top?

Why is my hand purple in the pic? I promise I'm not dead.

Thank you all for any suggestions

RL

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It looks like the plywood delaminated and with the bass bar comeing loose. Is the top sunk in because the bass bar support was removed when the plywood delaminated? How bad is the sinking? If the bass bar is clamped in place does the shape of the top return to original  acceptable shape? Possibly do a dry run and clamp the bass bar in place. Inspect the shape of the top with bass bar clamped. If this looks promising possibly then glue the bass bar in place with hot hide glue with same clamping on bass bar. Reassemple the bass. While apart re-glue any other loose top laminate layers and anything else loose on the inside. 

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18 minutes ago, Greg Sigworth said:

It looks like the plywood... anything else loose on the inside. 

Thank you kindly for the heads-up. I will be attentive to those things you mentioned.

When the strings were on, one side of the bass side f-hole would sink in about an inch at its worst.

Dry run clamping help a little but did not change much. The arching is shot... 

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No sarcasm intended, maybe you should be talking to a boatbuilder (or to an EAA aircraft fanatic).  All the fixes I know for delaminating plywood curvilinear plates involve pressing it in a mold while it's infiltrated with the glue of your choice.  Doing this up to luthiery standards is going to require a plaster cast and all that jazz.  Personally, I won't touch plywood fiddles of any size, but have done some of the other stuff.  :)

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Hi viola-licence-revoked

I wish you hadn't posted that picture. I have had a Made in Czechoslovakia bass near the bench for 4 years now - a friend begged me to work a miracle.

I took one look and told him it was firewood. He begged some more....

The belly was held on with panel pins, most joints were unglued, the plywood belly had long strips of the inner laminate hanging in loose strips, some bits were just fluttering around loose inside the belly. The neck was barely hanging on in the body.

My only hope is to carve a new belly and re-glue every joint - I repeat every joint!

Ah well - all experience is good.

Maybe I'll visit the local timber yard and see if they have a suitable spruce plank.

Oh - on your bass, start by removing one splinter of laminate from the body at a time. Do this by making a pad by folding a paper towel 2 or 3 times, wet the pad and press it over the splinter, wait 20 minutes and try lift the splinter away with the edge of a knife. If it doesn't lift off, replace the paper pad and wait a little longer. Let the splinter dry before gluing it back onto the belly.  Start at one end, then do the one in the middle. Limit yourself to doing only two an evening. The next two splinters will be those adjacent to the previous two. Doing it this way means that you minimise disturbing a "just glued" joint while working on the second one.

On re-gluing the loose strips I have never done that before so my advice would be as good as yours. Maybe make a long plastic sausage filled with fine sand to act as a glueing weight. Paint in hot hide glue, wipe the strip down into place using a scraper and place the "sausage" over over the loose strip. After an hour (when the glue has cooled off) I might remove the bag to clean away any excess glue - using hot water and a scraper. Then replace the bag and leave overnight.

Good Luck edi

Hi VdA - I have built cold laminated boats and repaired sailplanes both wooden and fibreglas :-)

cheers edi

Edited by edi malinaric

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15 minutes ago, Violadamore said:

No sarcasm intended, maybe you should be talking to a boatbuilder (or to an EAA aircraft fanatic).  All the fixes I know for delaminating plywood curvilinear plates involve pressing it in a mold while it's infiltrated with the glue of your choice.  Doing this up to luthiery standards is going to require a plaster cast and all that jazz.  Personally, I won't touch plywood fiddles of any size, but have done some of the other stuff.  :)

Thank you kindly for your suggestions

I'm looking into wood hardeners and zpoxy finishing resins...

I think I'm gonna try and soften the laminates first to rework the arch and force the bass bar back on. Once the bar is glued, flood the sucker with hardener or resin. That's my thinking now. Subject to change every 30 seconds...

The good thing here is I'm not an actual luthier, so I already know I can't meet luthier standards! ( Anyone out there hiring a hack?)

Hey, at least I got the kid a nice replacement carbon fiber endpin...

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10 minutes ago, edi malinaric said:

Hi viola-licence-revoked

I wish you hadn't posted that picture..

 

Hi edi

!! I feel your pain... Thank you very much for your input. Maybe I should make a new top too... Or I'll be gluing splinters for the next week and half...should let someone know in case I die on the bench and no one found out...

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Just now, viola_license_revoked said:

Hi edi

!! I feel your pain... Thank you very much for your input. Maybe I should make a new top too... Or I'll be gluing splinters for the next week and half...should let someone know in case I die on the bench and no one found out...

Mmmph - with your luck they'll just slip you into the bass, nail the top down and bury everything :-)

Negative on the new top - your one is recoverable. Use hot hide glue rather than looking at the modern adhesives. A thinner glue join for one thing. Greater grabbing power as it sets - after maybe 15 seconds. Hold things in position for that length of time with your fingers and then you can add the sausages.

Brian taught me to break a centre joint twice and re-gluing it before finally clamping it. Delay the sharp slap to break the joint by a couple of extra seconds and you then have to really give it a whack to separate the joint. Impressive how rapidly the joint strength increases.

On the arching - which side of the plate has subsided? Sneaking thought is that the first step would be to recover the arching - then do the laminations and splinters.

cheers edi

 

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25 minutes ago, edi malinaric said:

Mmmph - with your luck they'll just slip you into the bass, nail the top down and bury everything :-)

Ha ha ha. Really not a bad idea at all!

Roger that on the arching

running recoverbassarch.exe.....

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I would recommend clamping a 2x4 to each side of the plate across the widest part of the lower bouts and then

pushing wedges between the bass bar and the 2x4 until the arch is popped up enough.

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6 minutes ago, donbarzino said:

I would recommend clamping a 2x4 to each side of the plate across the widest part of the lower bouts and then

pushing wedges between the bass bar and the 2x4 until the arch is popped up enough.

Yes of course! Wedges! Thank you!

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5 hours ago, viola_license_revoked said:

Yes of course! Wedges! Thank you!

If you can find it of right dimension without too much cost, you may want to make them out of balsa wood, slightly larger than a harder wood, or make wedges and glue on hobby balsa strips as contact "shoes" . It smashes down nicely and take up minor differences here and there and is cushion like which helps avoid uneven pressure.

Seeing how this is what it is, and you don't want this repair reversible, just use smiths epoxy and be done with it

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6 hours ago, jezzupe said:

If you can find it of right dimension without too much cost, you may want to make them out of balsa wood, slightly larger than a harder wood, or make wedges and glue on hobby balsa strips as contact "shoes" . It smashes down nicely and take up minor differences here and there and is cushion like which helps avoid uneven pressure.

Seeing how this is what it is, and you don't want this repair reversible, just use smiths epoxy and be done with it

Yes I have a bunch of balsa wood I can use. Yeah reversibility is not really a priority I think ... At least not for the plywood. I don't ever need that to delaminate again. I don't think the sound of this bass can be improved or degraded by much. Thank you for your suggestions!!

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A few years ago I had to fix a laminated bass whose bass bar was unglued for about half its length resulting in a sunken top arch.  I made a clamping counterpart for the outside of the top from a piece of 2 by 4.  On the edge that would face the top I cut a concave curve in the shape of the desired top arch.  Then I reglued the bar to the top, pressing the top into the counterpart as I clamped the bar, to restore the top arch.

By the way, I think a consultation with a boat builder is not an outlandish idea.

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Are you sure it's plywood?  The grain isn't right for normal plywood, plus it looks like a matched right and left side.

LOL never mind.  I see the tears at the edges now.

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This might be the beginning of a long career in instrument work. When you get the instrument ready to assemble, while the thing is still open, it might be a good time to put some kind of a light sealer on the inside. Possibly vernice bianca , a coating of propolis, or something similar.  It isn't every day you get to work on the inside of one of these. Just a thought. 

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On 4/12/2019 at 5:15 PM, Greg Sigworth said:

This might be the beginning of a long career in instrument work. When you get the instrument ready to assemble, while the thing is still open, it might be a good time to put some kind of a light sealer on the inside. Possibly vernice bianca , a coating of propolis, or something similar.  It isn't every day you get to work on the inside of one of these. Just a thought. 

Thank you for those suggestions! I've never liked opening plywood basses but some sort of sealer might be necessary to help mitigate possible future delamination...

Here's the update:

run recoverbassarch.exe was successful. Now I can glue the bass bar down and hope for the best.

I flipped the top over and loosely clamped it to the sides to use as a cradle. Short 2x4's were clamped over standoff blocks and wedges were tapped into place after the rough area was wetted down with water. The whole thing was left to dry overnight and now the arch is essentially recovered. (Still a bit damp)

Next step would be to essentially repeat same operation with glue for the bass bar. Still a fair amount of work to go.

I'm going to dread gluing back the top. Without an overhang, it's going to take quite a bit of patience for a rank amateur like myself to line the edges up properly.

The varnish touch up operation all round the edge is going to a pain...

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Nice work. A couple years ago a repair shop south from here told me of the opportunity he had. A school system had a school bus load of Basses to fix that he worked on. The gluing of the top back on the bass with hot hide glue is not going to be easy. You have about 1 1/2 minutes before the glue jells and the joint will not be good. I have never glued something this big. Unless someone with experience tells you how to do it, I would clamp the whole top on dry with correct positioning of ribs and top edge. Doing this dry will allow you to make adjustments. Then unclamp about 6" at a time and work your way around. You can use small wedges to separate  to get the glue in. Use a artists pallet knife to apply the glue. You might want to glue the ends and corners first and then the bouts between next. A hair dryer can be used to heat the sections before you glue them which will increase the work time a little. Also the hair dryer can be used to heat the joint just after it is clamped which will help it to come together if there was separation. This will be interesting. I always get quite nervous when I glue. Use hot hide glue; other glues will disappoint and cause problems down the road. 

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