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Why shallac the inside?
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=27823		 Page 1 of 1
Author:  Chuck [ Sun Jun 13, 2010 9:40 pm ]
Post subject:  Why shallac the inside?
As I start my building I have been reading a lot of books some on classical construction and some on steel string construction. In a couple of the classical constructioin books the authors talk about putting shallac on the inside of the back and sides. I have not found any reference to this in the steel string books. What does the shallac on the inside do to sound on a classic guitar that it is sometimes done and why not do it also on a steel string guitar?
Chuck
Author:  Brock Poling [ Sun Jun 13, 2010 10:17 pm ]
Post subject:  Re: Why shallac the inside?
I have done this, but have since stopped.

The theory goes that when you seal the inside you retard the guitar's exposure to humidity fluctuations. Particularly if the plates are very light and prone to high environmental sensitivity. I suspect that classicals are built closer to the edge and steel strings are not... that is the reason this has not caught on with SS builders.

In practice I find that shellac is really hard to apply and have it look nice. It makes cocobolo bleed something terrible and turns your braces and linings orange, rosewood bleeds as well. Mahogany and Koa look ok, but it still creates a bit of a messy look. And when you consider the challenges this causes in future repairs it seems to me the negatives out weigh the positives.

I still think it is a good idea, but the practical realities of it make it "not worth it" in my book
Author:  Tim L [ Sun Jun 13, 2010 10:17 pm ]
Post subject:  Re: Why shallac the inside?
Chuck,

Somogyi refers to this in his new books and does recommend it for steel strings. The way I understand it, it seals the inside of the guitar from changes in humidity and makes the whole assembly a bit more stable. I am getting ready to do this to a 000 that I am preparing to close up and am planning on doing the top/back and sides with a light sealer coat. While I have never done it before this will be a guitar that I plan on having outside and traveling with.

Not sure how much that helps you out. Hopefully you will get a response from a classical builder who has more experience doing this.

Tim
Author:  Kent Chasson [ Mon Jun 14, 2010 12:46 am ]
Post subject:  Re: Why shallac the inside?
I usually just do my tops. It is a pain to do the back/sides and have it look good.

Shellac may be hygroscopic but the idea isn't to stop the exchange of water vapor with the wood. It's to slow it down and allow the wood more time to adjust to the change in stresses.
Author:  Jeff Highland [ Mon Jun 14, 2010 2:05 am ]
Post subject:  Re: Why shallac the inside?
I've done the tops on my last two (classical and Flamenco)
I figure it may provide some protection from cracking on those occasional hot dry windy days by slowing the moisture exchange.
Author:  David Malicky [ Mon Jun 14, 2010 2:12 am ]
Post subject:  Re: Why shallac the inside?
Page 12 has a great table:
http://www.taunton.com/promotions/excer ... ngWood.pdf
Note the test used a rather harsh 90% humidity for exposure. 1 coat of shellac makes a big difference for 1 day of exposure.

The original data is from here:
http://www.fpl.fs.fed.us/documnts/fplrp/fplrp462.pdf
#22 is orange shellac. #23 is white (clear) shellac -- Hoadley reports this one. #60 is "pigmented hat shellac" which performed even better, especially for 1 coat. I'm guessing it uses an artificial pigment, as orange shellac performed basically the same as white.

I would assume all the shellacs are waxed. I've not seen any numbers for dewaxed shellac, but I'd guess it would be worse.
Author:  runamuck [ Mon Jun 14, 2010 8:44 am ]
Post subject:  Re: Why shallac the inside?
I don't finish the insides of guitars but contrary to other opinions here, the fact is that shellac is one of the better finishes for slowing down moisture exchange.
Author:  Doug Powdrell [ Mon Jun 14, 2010 9:33 am ]
Post subject:  Re: Why shallac the inside?
As stated previous, to slow moisture transfer during environmental swings. I'm currently applying 1 coat to the sides and backs on a coupla classicals underway. Why? Not building in 'perfect' envir. conditions and don't anticipate guitars will live in 'perfect' conditions....

It's 'another step' (actually 2 due to the fact one needs to touch-up inside after box is assembled). and don't forget to NOT shellac glueing surfaces. I agree it could effect certain repair time to the interior......my $.02
Author:  Hupaand [ Mon Jun 14, 2010 10:20 am ]
Post subject:  Re: Why shallac the inside?
Nobody has any business looking inside anyway. Do what you want in there.
Author:  Chuck [ Mon Jun 14, 2010 11:08 am ]
Post subject:  Re: Why shallac the inside?
Filippo Morelli wrote:
Shellac is hygroscopic, so I'm not sure how these theories hold much water (pun intended).

As for the classical books, I'm assuming Bogdanovich's book is the one to which you are referring. The choice has less to do with classical guitars and likely more to do with his furniture making background :-) (just a small poke ... remember, I shellac the insides of mine ...)

Filippo


Yes Bogdanovich's book and Arthur Overholtzer's book Classic Guitar Making.

Thanks for all the responses.

Now I am wondering if there would be a difference between shallac and a clear base sealer. I put clear base sealer on some of the Northwest masks and paddles that I make after I have finished the carving and before I begin the painting. The clear base sealer does not leave much of a shine on the finish.

Again thanks for the references and all the comments.
Chuck
Author:  jfmckenna [ Mon Jun 14, 2010 3:14 pm ]
Post subject:  Re: Why shallac the inside?
What would that do to the smell... oh no I love the smell of the inside of a nice guitar :)

This is interesting I have always been told it's a big no no to put anything that resembles a finish on the inside of a guitar and I have always wondered why not? Makes perfect sense unless it harms the tone. I imagine that those of you that do this are careful do not get shellac on the back and top that mates up with the linings.
Author:  James W B [ Mon Jun 14, 2010 5:38 pm ]
Post subject:  Re: Why shallac the inside?
I do the same as Kent.
James
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