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Sides Warping After Nitro Lacquer


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hey y'all,

I built these dreadnoughts down in Arizona where the relative humidity remained a low 0-10% (constantly). When being built I kept them at a humidity of around 35-45%. My sides were approx. .085-.090" thick when bending. Now 6 months in the future the guitars have been sprayed (and cured), I moved back to my hometown in Nebraska where the humidity likes to stay at around 60-80% (when it's not raining). I started level sanding the lacquer and noticed the sides were warped (from taking on too much moisture).

Basically I have 2 questions:

1) What can I do to fix the problem I have?

Should I add some more vertical strips to help flatten (and stabilize) the sides, then shoot more lacquer?

2) How thick should I make my sides in the future?

replies much appreciated!

Thanks,

Mike

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hey y'all,

I built these dreadnoughts down in Arizona where the relative humidity remained a low 0-10% (constantly). When being built I kept them at a humidity of around 35-45%. My sides were approx. .085-.090" thick when bending. Now 6 months in the future the guitars have been sprayed (and cured), I moved back to my hometown in Nebraska where the humidity likes to stay at around 60-80% (when it's not raining). I started level sanding the lacquer and noticed the sides were warped (from taking on too much moisture).

Basically I have 2 questions:

1) What can I do to fix the problem I have?

Should I add some more vertical strips to help flatten (and stabilize) the sides, then shoot more lacquer?

2) How thick should I make my sides in the future?

replies much appreciated!

Thanks,

Mike

How much warpage are we talking about? Maybe a pic would be very helpful.

If you built and stored at 35-45% your wood was probably holding around 1 to 2% less moisture, if it was exposed to 0-10% it would be a much great change. Your guitar is going to continue to change throughout as seasons change. It is likely it will see 35-45% for a certain portion of the year and higher during other times of the year. You may be fighting something that is a going to be a natural occurance. Really it depends on how much the wood has moved(will tell you more about the degree of change). Since you are finishing it may be that you are being very critical of a small change. If that is the case I would set up your shop to similar building conditions, allow it to settle back to equalibrium, finish the job and then put it in service. Luckily overdrying after building in higher humidity is more dangerous. Building in a wetter shop then putting it in service in Arizona would have been bad.

Rich

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I would've liked to have a picture with my post, but my digital camera is out of commision for a couple of days.

Sadly this isn't the first time I have run into this problem. I've had this happen on another guitar, but fortunately it was before lacquer was sprayed. It's probly hard to understand my explination (I used the term warped very loosly). Basically the guitar is taking on a higher humidity now that I have moved. As I'm level sanding (getting ready for buffing) both of the sides just below the bouts, looks a little like this:

acousticsides.jpg

this is an exaggeration of course. The two outside flat spots look as though the kerfing has made prints in my finish. I'd say the dips are probly no more than .003-.005" deep. I have actually already sanded through the lacquer, so I definately need to spray more. I just wanted to know do you think I should reinforce it w/ a couple of vertical strips before I continue and fill them w/ lacquer?

and also is there any way to prevent this from happening in the future, b/c I plan to build for people all around the country. Should I make my sides thicker? maybe instead of 5 vertical strips to each side do 7 or 8?

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You know that looks odd to me. Are you sure your sides are .085-.090"(which is plenty thick BTW)-ish? It is sort of odd that even expansion would focus quite like that, as the box is not really restricted from expanding(getting deeper) short of a few little vertical braces, and the top and or back expansion would usually make the top dome out more. Has the tops dome expanded an extreamly large amount? Either way it seems like it would be hard for a top to focus so much stress on the sides to torque the kerfed lining quite like that. I wonder if the sides may have been thinned quite a bit when you scraped the binding? That could develop a thinner spot and make that area notably weaker.

Adding additional vertical side braces may add a bit of weight, but shouldn't hurt anything. If you plan to ship these all over the country/world. You have to shoot for a happy medium(which is what you mentioned you had done, 35-45%). It is always going to be safer to build with lower humidity than where it is going(but driving your build humidity too low can be a bad thing, remember what all the wood in Arizona looked like after it was in that environment for a few months). A lot of the way we build is done to contend with changes in moisture, because these thing will move(can't really stop that, unless you leave it in a humidity controlled bubble). Your not going to stop movement with bracing(although a little crack protection is a plus) or thicker sides. You should really try to anticipate movement and try to minimize the potential for focused stress when it inevitably happens(build it to allow for that shrinkage and expansion).

You should bring this topic up over at the OLF. There are a lot of much sharper people than me over there, and guys who are real pro's, not a weekend hack like myself :D .

Rich

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Fryovanni:

Most of my top braces are glued w/ a 30' radius form, and the soundboard was glued to the sides w/ a 15' radius form. We did this to help maximize volume and add tension. I made my braces pretty skimpy (thin), but when I glued them all up I made sure that they were all pretty tight. My back braces were glued w/ a 50' radius form, which the sides were sanded to the 50' radius form also (no tension built in on the back).

But like I said the dips arent very deep. I'm definately sure that the guitars sides were sanded to approx. .085" probly sanded .005" in the build process.. I wouldn't be suprized though if this particular section of the sides could be pushing .075". So I imagine like you said it could be a combination of things.

-sides being too thin

-humidity has caused them to expand (swell)

-the top and back are pulling the sides out of alignment

goodwood:

This is kinda the case. I remember when bending there were slight dips there before, but they were all sanded out. This isn't the first time I've run into this problem (like I said before). I fixed the problem on that first guitar, and just scanned it over the other day and noticed it has created little dips like these.

Personally I think what my biggest problem is, is the thickness of my sides throughout my build. B/c I remember when sanding the sides flat (after binding) there were dips like these. Also could be the unstableness of the wood before I started. I'm probly going to call some of my instructors and see what they have to say.

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I dont have a picture, but I use home made clear plastic bags (duct tape and 4mil poly drop cloths) and dessicant bags (ebay) when the humidity get too high, or when it WILL get too high I should say. This chart is fairly accurate and predictive:

http://merv.metr.ou.edu/weather/run12z/zz_usa_rhum.html

I wont be building anything for a few days at least.

You need to clamp down the sides to the steel bending form so it can't cup, that may be the issue. Ues metal clamps.

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Fryovanni:

Most of my top braces are glued w/ a 30' radius form, and the soundboard was glued to the sides w/ a 15' radius form. We did this to help maximize volume and add tension. I made my braces pretty skimpy (thin), but when I glued them all up I made sure that they were all pretty tight. My back braces were glued w/ a 50' radius form, which the sides were sanded to the 50' radius form also (no tension built in on the back).

But like I said the dips arent very deep. I'm definately sure that the guitars sides were sanded to approx. .085" probly sanded .005" in the build process.. I wouldn't be suprized though if this particular section of the sides could be pushing .075". So I imagine like you said it could be a combination of things.

-sides being too thin

-humidity has caused them to expand (swell)

-the top and back are pulling the sides out of alignment

goodwood:

This is kinda the case. I remember when bending there were slight dips there before, but they were all sanded out. This isn't the first time I've run into this problem (like I said before). I fixed the problem on that first guitar, and just scanned it over the other day and noticed it has created little dips like these.

Personally I think what my biggest problem is, is the thickness of my sides throughout my build. B/c I remember when sanding the sides flat (after binding) there were dips like these. Also could be the unstableness of the wood before I started. I'm probly going to call some of my instructors and see what they have to say.

Interesting that you build your backs so flat. I usually am either 25 or 28' on tops and I am very happy with 15' backs. Either way probably has little effect on your side situation.

I have built with sides that range between .65"(around tight cuts) and .90". No real issues(never had kerfed linings telegraph through a side). If this is a product of humidity changes, and the interaction caused buy the top and back effecting the sides. So be it, you will always have that kind of movement. If on the other had you sanded or scraped the sides very thin and the kerfed lining is telegraphing through that area. The only suggestion I have would be to try to keep things a little smoother during bending and assembly, so that doesn't require so much thinning.

Have you asked this question over at the OLF yet? That place is a pretty deep pool of experience.

Rich

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No, I haven't asked over there yet. I was going to wait a couple of days to get my camera back, so I can take pictures of it to help describe it better. I'll post the link here once it's on the OLF.

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