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Should You Seal The Inside Of Your Guitar?

What Works For Me & Why It’s So Important

Should you seal the inside of your guitar?  If you do seal it, what kind of sealer should you use and how should you apply it?  How does sealing the inside of your guitar affect the sound and how does it affect the long-term stability of the instrument and why?  

Have you been wondering about any of these questions? If so keep reading, we’re going to talk about all of them in this article.

If you’ve been searching around online you might have discovered that there are a lot of different opinions about whether or not you should seal the inside of the guitar body and what finish you should use, which one is going to be best?  I think that has a lot to do with the fact that we, as luthiers, each find different things that work best for us, and that’s totally fine. In this article, I’ll be explaining what works for me, how I approach it, and why I make all of the choices that I do. I hope it’s really helpful for you and that you’ll come away with some great tips and some good information.

You may or may not be familiar with the Luthier’s EDGE yet, it’s my online guitar making school. And one of the great things we do there is a monthly Q&A Session. I guess you could call it a webinar, live streaming here from my workshop where my students ask questions, and I help them solve issues, and deal with problems, fix mistakes, and make big decisions that they’re worried about and things like that. We have a great time. It’s one of my favorite things about the Luthier’s EDGE!   In one of our recent Jam Sessions a question was asked about sealing the inside of the guitar. I just thought it was such an important question! There are a lot of great concepts involved that spill over into other areas of guitar making. So I thought it would be a helpful thing to share with you guys.

The Question:

Do you ever put any kind of finish or sealer on any of the parts inside the guitar?

The Simple Answer:

Yes, and I’ll share 3 main reasons why it is so important below.

1 – Humidity: How Sealing the inside of the guitar protects the guitar and increases stability

Number one is humidity. There’s a general rule in woodworking, that whatever you do to one side of a board you’ve got to do to the other side of the board. Now there are probably several reasons for that, but one main reason for that has to do with humidity.

If I take  the guitar top for example and there’s no bracing on it or anything. Then I put a bunch of shellac or finish on the outside of it and leave the inside bare wood. When the humidity rises in the atmosphere, the part with the sealer on it, will absorb moisture, but at a much slower rate than the side of the wood that has no finish on it.  The side with no finish on it will expand and cause the wood to warp because there is an uneven absorption and loss of moisture on either side of that board. 

And,when it comes to arch top guitars, it’s really important to understand this behavior, because the arch top expresses humidity gains or losses by the top of the guitar either raising or lowering the action a surprisingly drastic amount (which is why most bridges have those thumb wheels, because it’s just so much of a change). So if you have a client going to play a gig and the humidity in the room is 20% higher than the environment that he normally keeps that guitar in the bridge could raise up quite a bit resulting in the action being way higher than what he/she is used to. 

When I seal the inside (top, back, sides, braces, etc.) of the guitar body, I’m slowing down and evening out that rate of loss or absorbtion of moisture and that’s going to protect against shock to the wood over time.  So humidity is reason number one that I always want to seal the inside of my guitars.

2 – Sound: How sealing inside the guitar affects the sound of the guitar

Reason number two is an interesting one, and that is sound.

The way that you treat that space inside the guitar, is like treating a room that you’re in.  When I was in college, I got a double major, one was in jazz guitar, and the other was in audio technology.   I had the awesome experience of studying with a guy named Bill Porter, who was the famous sound engineer, that did all of Elvis’ recordings and he worked with Roy Orbison and a lot of the famous guys of that era.  That was before all of the fancy tech that we have now, and he just knew how to listen to a room. I think that was one of the things that got me dialed in on my hearing and understanding what I was perceiving in a different way, a more clear way.

When I started making guitars, I heard about the humidity issue and so I started using albumin, which is egg whites, to seal the inside of my guitars.  Now the egg whites do seal the wood but, it leaves the surface with a rough texture. It doesn’t smooth out the surface or make it shiny.  So you can use albumin if you don’t want to create a reflective surface inside the guitar. There are some times that you want to keep that dry woody sound inside the guitar. It produces less echo and it’s more of this dry, rough woody voice, which is pretty cool in some cases. But there are other cases, that you might want to use shellac or something that’s going to produce a shiny surface and it’s going to really kick up the high frequencies inside the guitar. It’s like painting a room with gloss paint, or on the other hand putting a rough unfinished wood on a wall. 

So at this point we’re deciding, do we want to make a reflective room inside that guitar or do we want to make a non-reflective room in there? And it’s really fun to be able to control that.

In my case for the most part, I just like to seal the inside with shellac. I like the balance of the fact that I’m putting shellac on the outside and shellac on the inside. The only real subtlety that I do is on arch tops that are made out of maple. I do a heavier coat of shellac, because the maple already is less active in the higher frequencies and when I put the more shiny coat of shellac on the inside, it’s going to bump up the high frequency a little bit and it just gives the sound a little more sparkle and resonance. It also makes the bass sound a little more focused.  And it doesn’t just stay inside the guitar, it changes the voice a little bit.

But for Rosewood guitars like Cocobolo or something that’s got crazy high frequency action going on sonically in a classical or a steel string, more so on a steel string though, I’ll still use the shellac, but I’ll do a lighter coat. So, in that case, I’m trying to keep the wood more to the albumin or rougher wood affect. I’m not trying to add more to already busy trebles and an already reflective environment, but I still want to seal it.  These are just my perceptions or my opinions. It’s really important that you find the voice for your guitars, the sounds that you like with these different woods and the way you uniquely build.

3 – Aesthetics: How sealing inside the guitar makes it more beautiful

Reason number two is an interesting one, and that is sound.  

The way that you treat that space inside the guitar, is like treating a room that you’re in.  When I was in college, I got a double major, one was in jazz guitar, and the other was in audio technology.   I had the awesome experience of studying with a guy named Bill Porter, who was the famous sound engineer, that did all of Elvis’ recordings and he worked with Roy Orbison and a lot of the famous guys of that era.  That was before all of the fancy tech that we have now, and he just knew how to listen to a room. I think that was one of the things that got me dialed in on my hearing and understanding what I was perceiving in a different way, a more clear way.

When I started making guitars, I heard about the humidity issue and so I started using albumin, which is egg whites, to seal the inside of my guitars.  Now the egg whites do seal the wood but, it leaves the surface with a rough texture. It doesn’t smooth out the surface or make it shiny.  So you can use albumin if you don’t want to create a reflective surface inside the guitar. There are some times that you want to keep that dry woody sound inside the guitar. It produces less echo and it’s more of this dry, rough woody voice, which is pretty cool in some cases. But there are other cases, that you might want to use shellac or something that’s going to produce a shiny surface and it’s going to really kick up the high frequencies inside the guitar. It’s like painting a room with gloss paint, or on the other hand putting a rough unfinished wood on a wall. 

So at this point we’re deciding, do we want to make a reflective room inside that guitar or do we want to make a non-reflective room in there? And it’s really fun to be able to control that.

In my case for the most part, I just like to seal the inside with shellac. I like the balance of the fact that I’m putting shellac on the outside and shellac on the inside. The only real subtlety that I do is on arch tops that are made out of maple. I do a heavier coat of shellac, because the maple already is less active in the higher frequencies and when I put the more shiny coat of shellac on the inside, it’s going to bump up the high frequency a little bit and it just gives the sound a little more sparkle and resonance. It also makes the bass sound a little more focused.  And it doesn’t just stay inside the guitar, it changes the voice a little bit.

But for Rosewood guitars like Cocobolo or something that’s got crazy high frequency action going on sonically in a classical or a steel string, more so on a steel string though, I’ll still use the shellac, but I’ll do a lighter coat. So, in that case, I’m trying to keep the wood more to the albumin or rougher wood affect. I’m not trying to add more to already busy trebles and an already reflective environment, but I still want to seal it.  These are just my perceptions or my opinions. It’s really important that you find the voice for your guitars, the sounds that you like with these different woods and the way you uniquely build.

 I hope you found this article useful, maybe picked up a few tips and tricks, possibly even deepened your understanding of some of the factors at play in keeping your guitar stable and fine tuning that reflective or non-reflective space inside of the guitar body.

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