Sealing The Inside

I'm using rainbow poplar for backs and sides. I'd like to seal the poplar on the inside. Any drawbacks? Will the tone suffer? I'm using Stewmac's water based lacquer.

Usually, if you are going to seal the inside you would just apply a coat of shellac. You can use your own mixture (usually a 1 lb cut) or puchase some pre-mixed...either way, make sure to not shake it because you don't want the wax residue to float to the top. If you don't want to mess with that you can find spray shellac and be careful to apply a thin coat (no runs).

I usually just seal the braces and kerfing for a nice appearance...not really for any other reason. I build classical guitars so you can see a lot more of the inside. I don't coat the underside of the top wood or the back wood, but I know people that do (not on the top, just the back). I've never had any issues with this and there are no tonal differences that I can tell.

Disclaimer: I've never worked with poplar, though!

Although I can't remember his name, there was a guitar builder that was known for sealing the inside surfaces of his instruments. I had a request to do so once. I'm sure it made some kind of minute difference tonally, but I couldn't hear it.

Thanks a bunch guys. I'm gonna go with the spray shellac and give it a light coat.

Vic D said:

I'm gonna go with the spray shellac and give it a light coat.

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Vic, you can mix up some thinned shellac yourself and put it into a spray bottle - should work fine. We've used 50% basic shellac solution with 50% extra alcohol added and put it in an empty lense cleaner bottle we had.

Are you going to be spraying into the sound hole of an instrument that's already glued together? Or onto separate pieces before glueing. If the latter, remember to tape the seams before you start putting the shellac on.

Another thing I'm wondering about is why you want to seal the inside of the instrument in the first place?

Sprays have a real hard time making it into 90 degree joined edge corners, which the inside of a uke is full of. How about mixing up a thin coat and applying it with a foam brush?

jerickson said:

or puchase some pre-mixed...either way, make sure to not shake it because you don't want the wax residue to float to the top.

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Zinsser SealCoat is dewaxed shellac. It comes in a 3lb cut and there are directions on the can to cut it to 1lb or 2lb cut. Shelf life is three years. Good Stuff

koalohapaul said:

Although I can't remember his name, there was a guitar builder that was known for sealing the inside surfaces of his instruments.

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ignacio Fleta is the spanish maker associated with this
method

erich@muttcrew.net said:

Vic, you can mix up some thinned shellac yourself and put it into a spray bottle - should work fine. We've used 50% basic shellac solution with 50% extra alcohol added and put it in an empty lense cleaner bottle we had.

Are you going to be spraying into the sound hole of an instrument that's already glued together? Or onto separate pieces before glueing. If the latter, remember to tape the seams before you start putting the shellac on.

Another thing I'm wondering about is why you want to seal the inside of the instrument in the first place?

Click to expand...

I was thinking of just sealing the poplar but it's too much of a hassle right now. I'm probably wrong but my thinking was that it would slow the effect of humidity on the poplar and also maybe enhance the tone a bit. I'll probably expirement with it in the future.

While researching it I did come across an article that says laying a light coat of shellac on before spraying water based lacquer gives it a deep look similar to regular lacquer.

I don't know about "enhancing the tone" with the finish - I guess the camps are divided on this one. At least there are folks out there saying nitro lacquer, if applied correctly, improves the tone of an instrument (and even of a speaker cabinet), while others say the finish can never improve tonal qualities, it can only have a more or less negative effect (as in "thick coats of acrylic finish dampen more than thin coats of nitro or shellac"). So, again, I don't know if the inner sealing is going to improve the tone much. I guess you'll have to experiment with it to find out.

We'll be curious to hear what you report, both with respect to humidity balancing and tonal effects.

Merry Christmas,
Erich