Acoustic Guitar Construction in the small shop -- Kenneth Michael Guitars LLC

Information, ideas and help for those constructing scratch built acoustic guitars or acoustic guitar kits - © 2017
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PostPosted: Sun Nov 30, 2014 11:52 am 
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I have built this crossover guitar. It sat in the house for 4 weeks while we were in India. When we left a month ago the humidity in the house was probably around 4o to 45%. It is now around 55%.

The top is fan braced (9) but has no bridge patch. I'm pretty sure the top was okay when we first got home a week ago. The guitar was out until last Tuesday when my wife put it in a case. I took it out yesterday to set the bridge location, and I found the top caved in as in the picture I've drawn. It is still domed on the edges, but down the middle it has sunk in, probably around 1/8". My first thought was, this thing has dried out, but then the humidity is higher then when I built the guitar, so does that mean that this action of the top is due to too HIGH a humidity? I don't know which direction to go with it so for those of you guys who can tell me what is going on I'd sure appreciate the help. Thanks.



Kevin


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PostPosted: Sun Nov 30, 2014 12:47 pm 
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Location: Arnhem area, the Netherlands
Jee Kevin, I have no idea. Was the top domed (not doomed) in the first place or flat?
If it was concave at first it could get more concave while wetting.
I hope others have better thoughts.
Herman


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PostPosted: Sun Nov 30, 2014 1:25 pm 
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I've seen this happen to classic sound boards that were braced in a dish or solera, the main reason I glue on a flat surface. I think its the moisture in the glue having an affect on the plate. You mentioned that you "took it out" from where? Could be an equilibrium problem where there is more moisture on the inside of the guitar than in the room. Blow dry the inside a little and see what happens. $.02

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PostPosted: Sun Nov 30, 2014 3:50 pm 
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A fan brace classical guitar that does not have the bridge glued on has no lateral bracing in the lower bout. So it does not take much to have it go slightly concave or convex with humidity changes. My suggestion is to glue on the bridge, either flat or with a slight radius. The bridge and ultimately the string tension will pull the top back up.

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PostPosted: Sun Nov 30, 2014 3:54 pm 
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Location: Kalamazoo, MI
If the top expands (from adding moisture) it has only two directions it can go: up or down. Looks like yours went down.

Both John and Ken make good suggestions. Let us hope this works out for you. I bet it will.

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PostPosted: Sun Nov 30, 2014 10:56 pm 
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Thanks guys, it is encouraging to hear your responses. I kind of panicked. The only thing that made sense to me was higher humidity has caused this so I will go with that and see if I can restore it with a little help as Ken suggests, then store it in the case and see if I can dehumidify it some.
I am pretty sure that I will French polish this guitar.
John, I thought about the fact that the bridge needs to be on there to support that top in this area. If I glue the bridge on now will I have difficulty getting the shellac finish nice and level around the bridge? Anyone done it this way?


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PostPosted: Mon Dec 01, 2014 12:20 am 
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Brian Burns does it that way, you might drop him a line and ask him how he does it.

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PostPosted: Mon Dec 01, 2014 3:35 am 
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I have done it both ways. Easier without the bridge on.

When I had the bridge off, I would but a cotton rag with shellac around an eraser to get the areas close to the bridge. I have also mostly ignored the area but when I had a wet pad I would sweep in an lay down some shellac near the bridge. The second way is a little messy looking but if you get enough shellac on you can sand and polish that area.

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PostPosted: Tue Dec 02, 2014 12:38 am 
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I blew some warm air inside the guitar and it helped. Thanks for that suggestion Ken. I'd say the top came back up at least 50%. So I shot warm air into the case and closed it up. I'll leave it for a few days and see how it progresses. I'll get some desiccant to put in there too to hopefully continue the restored dome.


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PostPosted: Mon Dec 08, 2014 11:09 am 
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The humidity in the house is about 65. I have it 45 now in the case with the guitar. The top is almost back to where it needs to be, still a little dip behind the bridge area. I have a desiccant device in the case and will leave it there till spring I guess. I wanted to start French polishing it now. Oh well.


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