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Yeah - actually it's not worth even thinking about at this point. Nearly everything else is more important. Sand to 220 and be happy.

True greg lol.. just trying to do everything to the best i can for the first one wasnt sure if it was important or not (still not sure reading the replies )

Cheers anyway for the replies

Several things: First Cablepuller how about a first name even if it's not yours please? I'm affraid that I will slip up and call you the cable guy.....

Regarding keying true there are not a lot of uses for roughing up surfaces or "keying" prior to gluing in Lutherie but they do still exist AND it very much depends on the glue of choice for the application as well.

For example one builder who is very well known and knows his stuff more so than me or most here glues his bridges directy to a poly finish with CA and without removing the finish. Slight roughing up of the poly in the bridge foot print or keying.... helps keep the bridges in place. Not sure which, who's etc. CA he uses but I will be sure to ask the next time I see him.

If you laminate CF strips into your bracing and use ep*xy for the glue-up the CF benefits from being keyed a bit, sanded with perhaps 80 grit for just a minute and we benefit from wearing a high quality dust mask and cleaning up afterwards with a HEPA rated vac. I do this. So there are some places to key but you are correct that for HHG, Titebond original, fish and LMI no keying required.

Cablepuller my friend if you have noticed we have lots of scraper fans here. As such lots of folks have declared their preference to scraping over sanding. Why might this be in more specific terms?

The idea is that a freshly scraped wooden surface has more "joint energy".... than a sanded surface because at the moleculer level the wood cells are not as torn up and/or containing debris from the sanding process. More wood-to-wood contact since there is less debris in between.

Back to how very subjective it is in my view to advocate a level of smoothness inside the box. It's not just about how wood reflects. Much of what we hear from a stringed wooden instrument is the wood itself meaning that wood also absorbs vibration and transmits it as well. Reflectivity is only one of the plethora of difficult concepts that are somehow all mixed in together creating what we perceive to hear.

Quite frankly no matter how many models of how a guitar have been done, mathematical and otherwise I'm still in the camp that we do not really and completely know what's happening. At least I don't and I have no problem saying this either.

In fact it is this level of the known unknowns...... that helps keep me interested in so much as I rather doubt that even starting early with Lutherie that folks will ever really know what has some of the impacts on what we perceive to hear and what does not.

We get clients frequently who want to pay $20 for a cap and believe that they hear a difference.... Yeah those electrons do tend to care what color that cap is too....

So Cablepuller here is another sugestion for you that may help - that's the intent. On the Stew-Mac site in the pages about their kits the dreadn*ught kit used to have the assembly instructions available as a PDF and free for down load. It's an interesting read and some methods are very valid, others not so much but it gives you something to read in the tiled library if you want...

Hesh thankyou

Thats nice to hear that there is some unknown with the end result even with you experienced luthiers..starting to think i should have posted this on the kit forum as some seem a bit ruffled but most are very helpful

Loving the build and i know its a kit but really want to add as much quality as i can (sound more than aesthetics) its what i do
Theres masses to learn about the theory
Will take on board your advice and thanks

Is that online somewhere? I'd love to see some research done on the topic.

The only thing that I can add to this discussion is that I've played Sergei de Jonge's guitars, and I've interviewed him, and I'd be inclined to listen to anything he has to say.

Steve

One of the properties makers look for in guitar wood, especial top wood, is a high ratio of stiffness to density. If you take a piece of wood, straight off the sander with, say, 60 grit and measure its Young's modulus you get a different result from the same piece of wood planed to thickness. The planed wood is always stiffer. The reason for this is that at 60 grit, you can get sanding marks up to ~0.5mm deep in the surface, essentially a surface with totally mashed wood fibres, incapable of taking any load. So the majority of that 0.5mm thickness (a large amount in the scheme of things) is adding mass but not much stiffness and in a dynamic test of wood properties it will measure up with significantly lower Youngs modulus. So if you're looking for light and stiff, planed wood (which should have a glass-like finish if you're any good at planing) is the way to go, because it is always stiffer at the same thickness than sanded wood.

Consider this: if you bind your guitars using tape to hold the binding in place whilst the glue sets up, you'll likely know that it's easy to pull fibres out of the top wood when you remove the tape. If you shellac the top first, hardly any or no fibres pull out. What this tells you is that shellac (amongst other things) binds the wood fibres together which has the side effect of making the wood stiffer. I find it's worth 2-3 Hz or there abouts lift in the main top resonant frequency if I shellac the inside of a guitar.

Roughened vs smooth gluing surfaces. Back in the day, many brands of glue had instructions on the packaging that exhorted you to "roughen the surface with sandpaper" or words to similar effect. What really matters is having new surface, which has high surface energy (before it all oxidises) and which is more reactive with the glue and so forms stronger bonds. How you create that new surface is largely irrelevant, so long as the gluing surface is new, clean, smooth and the surfaces to be glued have high conformity. Planing is an excellent way to get there.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

I tried to find it when this subject came up a year or two back, and could not. I think it's gone. It was some old engineer guy who was designing horn (speakers). The reflectivity of materials as you go around corners is critical to their design. There were nice frequency response graphs and everything. There may be other info on this from that same field.

New person to the forum here but I'll put my 2 cents worth in. Jean Larrivee once said in an interview that sanding to 80 grit on the interior of the box is all that is needed on a steel string guitar. Personally, I go to 150 on the thickness sander with no finish. I've played great guitars that were varnished on the inside and great guitars that looked like a lawnmower could be used inside (bit of an stretch there but it gets the point across). I think that as a builder it is more of a personal style question than a definitive point of sound and tone. Attention to the inside, to some folk, is equated to superior craftmanship, like buying a car with ebony veneer on your ashtray. You don't need it, but, it looks good.

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Blue Ring Acoustics
blueringacoustics.ca

Good point greg b..used to buy hifi magazine and the interiors of the speakers were important for mid and base response (if i remember right)

Tysam reading all the comments by people with loads of experience it probably is more down to preference than essential to smooth it up for sound, but would be good to know for sure

What do the big boys do? ie martin, faith etc

Anyway cant stop Just of to polish my ashtray

80 grit on a 36" widebelt timesaver leaves a much different finish than on a drum sander...

Trevor I don't think that your example is very applicable because no one that I know or know of is currently sanding the insides of their boxes with 60 grit and calling it a day.... Instead many of us go to 220 or even stay at 120 and although I'm a fan of scraping too uber stiffness is not always desirable every single place in or on a guitar. It's a system.... and I think that you've said that before too and I know that I have.

Back to the iconic stuff I heard a story today about a high-end restoration of a valuable guitar in which the folks restoring it wanted to duplicate the materials and methods originally used. As such they did testing to duplicate sanding scratches and found 80 grit to be about right.

It's been mentioned that there are lots of guitars still out here worth far more than most of our vehicles that are rough inside, have saw marks etc.

Generally speaking the data and examples are all over the place and just like many things in Lutherie it sure would be nice if it became more politically correct to simply say that we don't know because we really don't.

Snake oil abounds in the trade from the snot-nosed punk paying $20 for a capacitor because we all know that those "vintage electrons" sound better.... to folks who can hear a difference in the sandpaper the maker used....

And maybe that's why we build.... to find out... eh.

Not sure it's been brought up before yet in this thread but what about market expectations and what prospective clients (remember them...) actually want? Show them a guitar that inside the sound hole it looks like it was put together in a second grade crafts class and I wonder how well that will sell....

At least in my case I tidy up the insides of mine 1) because I can and 2) because in my view that's the expectation and 3) it's a sign of the dedication and devotion of the maker to never, never let anything leave home that you are not proud as can be over.

And lastly... for very nearly as log as there have been guitars some very bright folks have attempted to mathematically model them, use technology, mass spectrometry even and so far no one has really in my view achieved a fairly complete understanding of exactly all of the the things that are going on including where, why, how, etc. And that my friends keeps it all very, very interesting but also strongly indicates that we often just do not know. No shame in that in my view.

Things that make you go hmmmmm What I got from Trevors post was "if" your good with a plane thats about as smooth a surface as you can get, right? I've gone from cutting the radius on bracing stock with a Luthier Suppliers jig then hitting it with a very sharp block plane for the gluing surface, to just doing the entire job with a plane. My next build its hand plane city....

Well since Trevor was the only person to show up here with hard data, save for maybe some French Gypsy, I wouldn't be so dismissive.

Trevor was primarily talking about top stiffness, and more than thrice I have called it a day at 80 grit. I don't know anyone besides Eugene Clark that doesn't want more top stiffness, so I think it's very applicable.

I have been planing my soundboards to thickness because I think it's more accurate than sanding, but I usually end up sanding the inside before I glue the braces! Maybe in the future I will try to be more careful and go straight from plane to gluing.

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Pat

I am in the planing camp too: from planing to gluing.
With a good tuned plane, the top will shine under light after planing.
There is really no need for further sanding for me.

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Sen CL Goh
http://senguitar.blogspot.com

A bunch of years ago Rick Turner spoke to this exact topic here on the forum. In the crass, to the point lingo he was known for using on the forum, "anything more than 150 inside the box is wanking."

It was his way of reminding builders to give attention to things that were really important. But at the same time giving permission to those builders who really got a kick out spending time on every tiny detail, even if it wasn't needed for a good instrument.

So if sanding up to 1200 or shellacing the inside of the box floats your boat, go for it.

I have to chalk this one up to another case of "Princess and the Pea".

Sanding or scraping to the same final dimensions or the same final stiffness, what would the overall final changes be? A percent of a percent of a percent? Same for acoustic damping in a complicated machine containing so many other factors which could exponentially outweigh any influences here.

If it were possible to construct two identical bodies so precisely consistent with this being the only change, I highly doubt there are ears on this earth which could reliably detect a difference between them in a blind test.

Glue joints, yes, it can affect a minor difference depending on the degrees of roughness we are talking about (keying is bad, planed or scraped best, 180 grit sanded - just fine in all practical terms). Clean work not only speaks of fine craftsmanship, but arguably builds/reinforces an attitude of uncompromising standards which tends to carry through to other aspects of the build, thereby indirectly contributing to other qualities. As to effects of final surface texture on acoustics though, I'd say it's a pea laid under a dozen mattresses.

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Eschew obfuscation, espouse elucidation.

Hard data.... I must have missed it both the other day and upon further review just now.

Dismissive is not the word that I would use to describe my own intent, after all my own intent is actually mine so I should know.... Wanna argue about that?

Instead I don't agree with Trevor, no biggy, it happens all the time... The notion that a scraped surface is noticibly.... stiffer than a sanded surface of, and here's the kicker.... the very same final dimension is what I don't agree with. I still don't see the data either not that I am asking.

To be sure too since this post here is also mine.... not only is my intent to discuss freely which may or may not mean that I might agree in a topic with another industry pro but never to be dismissive.... That is a "quality" that I am not known for unless someone is a total arse and Trevor most certainly is no arse.

Back on topic. I also was considering the sides, back, bracing - not just the top where stiffness is likely most desirable although I like it in my bracing as well.

Moving on to Rick, he does not mince words, Turner: My recolection of some of his very valued rants was not only was he saying that you guys stress to much over stuff that is mouse nuts in the bigger picture he also said, over and over again, that repair experience should be very valued as well. I took this so very much to heart that I changed my life over it....

Rick was real big on servicability AND the notion that the final set-up is FAR more important than what glue someone used.... or if we scrape or sand... I'll add that the set-up is the human interface and step one in the bonding process. If you want to sell an instrument or have the user enjoy the instrument or enjoy it yourself learning to set it up properly AND build condusive to being set-up properly is far more important than what the inside of the box looks like.

Back to the OP, remember him... this discussion is not going to get you where you want to go. Start building!

I said planed.

Do you disagree on principle or have you done any measurements? You'll only convince yourself by measuring it yourself. Try it.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

OK planed - still the difference is mouse nuts and nothing to stress over IMO.

Measurements would be easy to do if one was concerned over this issue. I'm not concerned again believing that in the context of a thread started by a new builder who has not built anything guitar wise yet what a black hole this entire discussion is....

I would be happy to do some testing if my days were not filled with let's see two set-ups a Martin and a Gibson, a fret dress from hell and pick guard crack, removal, finish the wood under the new guard with another Martin, and an archtop that needed it's top glued back on and a crack fixed. That's todays total and I'm still at it - you wanna do the testing Trevor.....?

Sorry I forgot the Guild D-25 from hell too (a black one as well, my favorite....) that I took the bridge off this morning and we will make a new one likely tomorrow.

So no need to convince me it's not an issue dear to my heart or wallet.

I have a 1980 Kohno 30 Classical guitar. This is a high end model that sold for around $9,000 when Kohno died several years ago. The inside back is rough, apparently the result of going through a thickness sander with rough grit and not being smoothed in any way afterwards. The back, and I think sides, but not top, are finished inside.

I use a Plane but even if you use a sanding machine it literally takes a few minutes with a card scraper to remove those kind of scratches. Even 120G scratches appear ugly to my eyes. A scraped surface might not finish perfect but it's more of a craftsman type approach.

Just an opinion, then. The reason for being pedantic about planed is because I've tested that. I haven't tested scraped, so I don't know how scraped performs, though I could give an opinion, for what it's worth.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

No problem Trevor please be as pedantic as you wish. Personally long-winded is more my style....

Happy Holidays!

Trevor,

I for one would be very interested in your opinion about scraping.

I have "issues" with sanding wood so I use my scrapers any time I can and save the paper for times when I must use it. Specifically hand sanding.... For some strange reason it has the same effect on me as scraping fingernails on a blackboard to some people.

So I use my scrapers and hand planes most of the time. Besides I would rather make shavings than dust

Cheers,
Bob