finish INSIDE a guitar

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a_arnold

Posts: 558
Joined: Jul. 30 2006

finish INSIDE a guitar

Anybody ever heard of a luthier putting a finish on the inside of a guitar?

I've heard one opinion expressed (in Huber's book) that loudness is mostly a tradeoff between stiffness and mass. The lighter and stiffer, the better. Lighter means more responsive (soundboard accelerates faster in response to string vibration) and stiffer means the whole soundboard responds -- which is why the slight arch in the Torres design adds loudness. Trouble is, too thin will reduce stiffness, so there is an optimum, which some say is achieved by those 2-layer tops with the routed space between.

So wouldn't a good hard finish on the inside contribute to the stiffness? And it would certainly reflect sound better. Surely this has been tried . . .? So it follows that there must be a reason it isn't done regularly?

Tony A.

REPORT THIS POST AS INAPPROPRIATE | Date Apr. 16 2007 18:43:43

Per Hallgren

Posts: 241
Joined: Jul. 1 2006
From: Sweden

RE: finish INSIDE a guitar (in reply to a_arnold)

First, I object to that loudness is MOSTLY a trade off between stiffness and mass. Weight is important but not THAT important. My first thing on the list is the correct balance between all the design elements, where weight is only one factor.

One very important reason to not put a hard finish on the inside of the soundboard is that it would be much more difficult to repair cracks in the soundboard. At least with conventional glues.

REPORT THIS POST AS INAPPROPRIATE | Date Apr. 18 2007 9:59:32

aarongreen

Posts: 367
Joined: Jan. 16 2004

RE: finish INSIDE a guitar (in reply to a_arnold)

I think your book is way oversimplifing things. All things must work in balance to produce an efficient system, no matter what the system is. To say lighter is better is not untrue, but its not true either. Same goes with stiffness. The concept of "reflecting" sound has little to do with how guitars work, IMO.

Finish adds damping, one reason we talk about the benefits of a thin finish is to minimize this. Thats not to say I believe a guitar without finish would sound better than the same guitar with finish (depending on the finish and application). However to finish the inside for acoustical gains would be most likely a fruitless endeavour. Finishing the inside of a guitar is a holdover from a common furniture making practice. Since the guitar is a mostly closed box with all the end grain nicely sealed up by the bindings, the inside of the guitar is pretty well protected and it will expand and contract at the same rate as the finished outside.

As Per correctly points out, when the inside of a guitar is finished, it makes life difficult for future repairs. I am dealing with one right now, an older Kohno, the rosewood back had a nice crack in it that I managed to get to close. Thanksfully the crack is mostly within reach and the inside of the guitar has only a light shellac sealer. It was still tricky to do a clean job scraping off the finish so I could glue in the cleats.

REPORT THIS POST AS INAPPROPRIATE | Date Apr. 18 2007 12:18:17

a_arnold

Posts: 558
Joined: Jul. 30 2006

RE: finish INSIDE a guitar (in reply to a_arnold)

Thanks for the answer, you guys. I'm sure you're right, Per, about the mass/stiffness issue. The author of that book (Huber) isn't a luthier, although he seems to have talked to a lot of them. Maybe I misunderstood him.

Aaron -- are you saying Kohno's have a finish on the inside? I've never had my hands on one. I didn't know that. Nor have I ever seen a guitar with a finish inside, but you both (Per and Aaron) seem to have encountered them. Do other makers do that too? What do they give for a reason?

Tony A.

REPORT THIS POST AS INAPPROPRIATE | Date Apr. 18 2007 14:34:36

a_arnold

Posts: 558
Joined: Jul. 30 2006

RE: finish INSIDE a guitar (in reply to a_arnold)

Hmm. I just saw a Kohno (classical) that is finished on the inside.

Tony A.

REPORT THIS POST AS INAPPROPRIATE | Date Jul. 15 2007 4:14:00

TANúñez

Posts: 2559
Joined: Jul. 10 2003
From: TEXAS

RE: finish INSIDE a guitar (in reply to a_arnold)

It was going to be a flamenco, after the finish was put on the inside, it became a classical.

_____________________________

Tom Núñez
www.instagram.com/tanunezguitars

REPORT THIS POST AS INAPPROPRIATE | Date Jul. 15 2007 15:17:03

stephen hill

Posts: 300
Joined: Feb. 16 2004
From: La Herradura, Granada, Spain

RE: finish INSIDE a guitar (in reply to a_arnold)

pity to varnish inside as it also cuts out the smell of the cypress on a flamenco. some makers varnish the bars and center strip. Fleta used to varnish inside but as aaron points out, repairs are harder to do later. also it depends on how you build as varnishing the back and sides when using the spanish method of construction is hard.. using the 'french' method where you build the back and sides first and then put the top on is easier. No point, IMO

_____________________________

stephen hill - granada spain
http://www.spanishguitars.co.uk

REPORT THIS POST AS INAPPROPRIATE | Date Jul. 16 2007 8:46:56