Disaster

[blackalex1952]

I have been rebuilding a dreadnought guitar. It requires new back and sides. The soundboard has been sitting in my dehumidifier for a week or so. I'm annoyed with myself for not thinking the process through properly, as the braced soundboard, which is coated with a lacquer on the outside, has cupped badly. The finish side is the concave side, and is obviously some kind of polyurethane. I suspect that the problem wouldn't have been so bad if the coating was nitrocellulose, because I believe that the thickness of the finish and the fact that nitro 'breathes' a bit would have helped. Maybe I should have made up a clamping frame to stabilise things, at least to hold the soundboard by the edges, and taken the RH down a lot more slowly.
So the question is, what to do? Should I try steaming the wood? I'm concerned that the steam will penetrate the non coated inside of the soundboard and braces and expand that side even more and finish the whole thing off for good! Also worried about lifting the finish. If the soundboard can be straightened, what is the best way to then lower the moisture content for reassembly to the new sides without repeating the problem? My thinking at this stage is to make up a support frame then put it back in the low RH conditions, and sweat on the finish coping...One other point, the guitar is a Martin and has quite a flat soundboard anyway. I'm concerned that if I do get this top back to a useable state and complete the rebuild that it will cup inward down the track if it gets exposed to low humidity level in its future life. I have been building to 20 and 25 ft top radii, this is less as I measured it before my "mistake".
I think it best to ask the collective on the forum here for advice!
Help! -Ross

[kiwigeo]

Don't steam the wood...

What's the humidity like in your shop? If it's 45-50%RH then I'd leave the soundboard on a go bar deck with a regularly spaced rods to hold it down and leave it for a few days and see what happens.

What sort of humidity is your dehumidifier running at?

[blackalex1952]

Humidifier is at 45%, may have been 40% at one stage, it's not that accurate a machine, I hope to get a separate controller and hack the inbult electronics- it's a Delonghi CF08E. What is the reasoning behind your don't steam thinking? The board is so warped that go bar pressure would break something, I'm sure. It is fully braced. If I put the soundboard under pressure, I think I will have to do it gradually, thinking about a clamping jig. The humidity in the shop is around 70% here in the Alps.
Lesson- approach humidity with humility-R

[kiwigeo]

Humidifier is at 45%, may have been 40% at one stage, it's not that accurate a machine, I hope to get a separate controller and hack the inbult electronics- it's a Delonghi CF08E. What is the reasoning behind your don't steam thinking? The board is so warped that go bar pressure would break something, I'm sure. It is fully braced. If I put the soundboard under pressure, I think I will have to do it gradually, thinking about a clamping jig. The humidity in the shop is around 70% here in the Alps.
Lesson- approach humidity with humility-R

I suspect your dehumidifier is running at lower than assumed RH. Go bars apply alot less pressure than a clamp.....from memory the figure is 7-14lbs. How long was the guitar sitting in your shop (and 70%RH) before you put the top into the dehumidifier? Id still try letting the top at workshop humidity on a go bar deck for a day or two and see what happens. I'd then look at gradually bringing humidity down before re-fixing the top to the guitar.

[simso]

My process would be a little different,

The bowing is extreme to say the least, there is a serious problem happening. If its been humidified properly then I would proceed as follows

I would remove all braces and bridge, and then flat clamp or clamp in a radius caul to match new top radius.

Reshape the braces or fit new ones and the bridge to the new profile and then refit, anything else and I think you will suffer from it when finished and strung back up

Steve

[blackalex1952]

Goodness me!!! This is a classic example of the "road to hell being paved with good intentions"...I would rather build, then the lessons are on my own time...good experience though if I succeed. I've managed to load it into my gobar deck with a little tension for now...flexible 10mm cedar lathes following a 25ft radius board, not under full go bar tension yet. I wrapped plastic around the deck for possible later humidity increase with moderate steam. I'll see how I go after a few days. If this idea fails, I guess new bracing is the go.. if I have to go to all that trouble, I may as well make a whole new soundboard given that I have to finish back and sides...may as well respray the whole guitar.
In that case the only thing original will be the neck and bridge and I will have paid dearly for not thinking things through first!!!!
Thanks for chiming in guys, you blokes are more than generous, given that you are obviously busy people. If ever we meet the beers are on me! -R

[kiwigeo]

Thanks for chiming in guys, you blokes are more than generous, given that you are obviously busy people. If ever we meet the beers are on me! -R

Steve would be busy...me, unless the price of oil gets past $100 I'm not busy

The problems you're having can be frustrating but in the process of solving the problem you learn alot and add to your total knowledge which can only help with future builds/repair jobs.

[frankfalbo]

Not sure if I'm missing something here, but in order for the top to cup like that, it would mean the dry side is the side under the polyurethane. If the poly was the wet side, it would cup the other direction. As simso suggests, this seems more like a mismatch between the braces and the top, or that the guitar was built at a lower humidity level than the one you've created with your dehumidifier.

If it's a temporary mismatch, then eventually the top and braces will even out, and you are right for this to occur under gobar tension. If it's a permanent mismatch, in that under a more extreme condition the top would return to flat, but at 40-45% there is argument between the two, then It would unfortunately require a more nuclear solution, to reset the glue relationship between the braces and the top.

I would wait it out under gobar(s) for a week or two.

[simso]

[Steve would be busy...me, unless the price of oil gets past $100 I'm not busy

Bwahahaha, if only you knew.

For info I do Woodwind and Brasswind repairs as well as Guitars, I am just basically a glorified mechanic.

Todays count, as Christmas workload from schools is coming in, is 364 instruments in for repair, WOOHOO, actually more like woo, as I am the one who that has to do it all, and done by start of February, again woooo, no hoooo.

Steve

[simso]

Goodness me!!! This is a classic example of the "road to hell being paved with good intentions"...

It happens from time to time.

To make you feel better, I have a guitar here doing the same, customer knows of the issues, so all good.

It came in with slight collapsing of the top, it needed a new x brace - Late 60's guitar

So I pulled the back and it tore out as it was a laminated back (really - manufacturers you couldnt have used normal wood)
I replaced the X brace - all good
I refitted the back and found the sides had also torn out (they are a laminate as well) - still got to fix them
I rebound the back to find the fretboard was actually broken in half at the 12th fret so I had to pull and replace the fretboard
When the fretboard came off, I found the neck block was broken away from the sides
When I fixed that I found the neck without fretboard fitted was severley warped (12 string)
I re-inforced the neck with two carbon fibre rods
I had to shape a new ebony fretboard and stupidly slipped whilst sanding, slipped and sanded my fingers and knuckles which put me further behind
I had to make another ebony fretboard and fretslot and shape size it.

I now have a new fretboard fitted, I have to fit pearls and fret it, then fix any cosmetic issues and the tear out of the sides.

It happens, trust me, even after doing it for many years it still sometimes happens.

Steve

[kiwigeo]

Disasters part and parcel of guitar making....

Got a parlour guitar finished the other day. All going well until I discovered the truss rod wouldn't turn. Ripped off the fretboard and extricated the epoxied up up truss rod. Decided to take off the headstock veneer while I was at it as I wasn't happy with some tear out around the tuner slots.

[blackalex1952]

I have left the soundboard under go bar pressure overnight, with a tub of water on the deck as well. The deck itself is enclosed in plastic. The RH this morning was 65% and the tension is slowly leaving the timber, I was able to increase the go bar pressure today. The bridge is still on the guitar, I put 10mm high flexible lathes between the radius dish and the soundboard. That would mean that the radius of the 25 ft dish has changed, but I'm not too worried about that. If this works, having disabled the go bar deck for other work for a week or so, how to get the RH back down successfully before closing the box without the problem re appearing? I'm beginning to wonder if the issue may have been caused by some glue creep between the braces and the soundboard. Or perhaps the humidifier took the moisture content down too quickly on the unfinished side, thus setting up case hardening and a steep moisture gradient which caused the problem.
It's quite weird given the cupping on the finish side of the top, one would expect that at 45% the curve would have been in the opposite direction, and that re humidifying would make the problem even worse! I'll just have to wait and see....At least I have ordered a humidity control sensor and switch, and will hack the electronics of the de humidifier so it can be controlled externally.
Maybe we need a luthiers disaster stories thread? Hopefully with the occasional happy ending? Someone might get inspired to write a country song.... -----------------------------------R

[blackalex1952]

So here is one of my disaster stories with a happy ending...A friend asked me to get them a cheap but good archtop. I found on a website, I won't name and shame the operator, a Levin archtop. These are great value for money given the sound. It was made around 1951. The photos and description showed that the guitar had been repaired by the website owner, along with testimonials from people regarding the quality of his work and service. He has many other instruments for sale. He had installed a vintage neck pickup. The guitar looked a bit rough but was correspondingly cheap. He offered a 24hr approval service, purchaser pays the reverse courier fee if rejected. My friend was keen to have the guitar sent directly to her rather than to me for a look over. It arrived promptly from QLD (to Victoria). She then took it to a very well known and respected luthier/repairer in Melbourne for a look over. This guy talked her into a fret job and new strings along with what was supposed to be a thorough inspection and minor crack repair for $300. She then noticed some rattling and asked me to look at it when I next visited old Melbourne town. Because she had gone and got $300 worth of work done on the thing rather than sending the guitar back to the seller, she felt she had no recourse. What I found was.....
The side crack repair had been done but other cracks had been missed. The pickup rattled. The potentiometers rattled. The wires rattled against the back. The neck was straight and a very nice fret job had been executed. The bridge base plate had been shaved too thin in order to lower the action, as had the bottom of the bridge which meant that the bridge had split along it's length. This had been done because of the change in neck angle. The neck angle wasn't quite right, I then examined the soundboard cracks either side of the pickup which had been routed into the top. On removing the pickup I discovered that the single diagonal soundboard brace had been sawn through in order to install the pickup originally. The side of the brace had been repaired with a thick backsawn mountain ash lollypop stick which had been badly glued to the side of the brace in an attempt to bridge the cut. When this was pressed on the gap in the cut opened and closed, so the splint was totally ineffective, and had caused the neck angle to lean, in fact flex under mild pressure. This is why the bridge had been shaved to a point where it collapsed.
So I removed the brace splint, capped the brace with spruce and filled the cut with a sliver of spruce. I then inlaid a piece of spruce into the soundboard, colour matched, sprayed and re installed the pickup which was in fact a surface mount. The hole had been made for a single coil pup prior to the current pup, which merely covered the hole. I made and fitted a new bridge. I repaired and cleated a few cracks, fixed the rattles.The neck angle had sorted itself out somewhat after the soundboard had been repaired, and the guitar now plays and sounds great. I did this at mates rate, as I felt responsible for getting her the guitar in the first place, even though it was her mistake not to take the vendor to task ...

[frankfalbo]

I'm beginning to wonder if the issue may have been caused by some glue creep between the braces and the soundboard...It's quite weird given the cupping on the finish side of the top, one would expect that at 45% the curve would have been in the opposite direction, and that re humidifying would make the problem even worse! I'll just have to wait and see...

Yeah that's what I was thinking. There is some argument between the braces and the top. That could be why more humidity is restoring the relationship between the two. Like the moisture content of the braces was not equal to the top when they were glued originally. Either way, take your time.

The one thing to take solace, is that if it's flat (or domed) when you install it onto the sides, then this inverted shape is less likely to return over time considering there is string tension on the top.

[blackalex1952]

Thanks everybody for the input. I go barred the top for a week in a 25 ft dish, with the lathes I previously mentioned to give bridge clearance and keep some sort of radius on the top, having enclosed the go bar deck in plastic. At first I introduced steam into the enclosed go bar deck to raise the RH and moisturise the wood-radical and risky, I know. After a few hours at 85% RH, I then used bathroom heat lamps (radiant heat) to slowly penetrate the wood the theory being to heat the glue joins between the braces and the soundboard, and to lower the moisture content in the wood. A surface thermometer was used to give a surface temp of around 150F. The lamps were placed at a distance that held this at a stable temperature. ( If it had been a horse it would have been happy...but that's a horse of a different colour -probably charcoal!) A few hours and the RH was 40%. Over this heating and dehumidifying process, I slowly increased the go bar pressure to get the centre of the soundboard, ie the X braced area where the braces are the thickest, to follow the full curve of the dish.The reason I took the gamble on the steam first was to get the timbers to be a little more compliant, an it is my understanding that the heat would penetrate the braces faster and more thoroughly due to the moisture. Lamps turned off and the enclosure left for a week. The RH had climbed back up to 65% by the following morning. Success, so far, there is no room for the deck in my under bench dehumidifier set up, so just before glue up I will lower the moisture content with the heat lamps, but will try to keep the surface temperature much lower than before. Hoping this will work.

The one thing to take solace, is that if it's flat (or domed) when you install it onto the sides, then this inverted shape is less likely to return over time considering there is string tension on the top.

Well, string tension on the tail side, and compression on the soundhole side...unless your bridge design...? But it's a Martin, and factory overbuilt.....Thanks Frank Falbo, you certainly got me thinking about glue creep, but I will believe this when I have the box re assembled and under normal environmental conditions for a while. I also don't know what the top will be like when I take the RH back down to 40-45% until I remove it from the go bar deck at that moisture content. Scary stuff! But luthierie is living on a sharp edge! That goes for tools, tolerances, lightness of build, customer patience etc... and finances it seems!!!
-Ross