Official Luthiers Forum!

What finess grit do you take the inside of the body to? Any reasons as to the thinking is also appreciated.
Thanks,
Bill

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Formerly know as Mandodiddle.

Bill my friend I sand the inside of my boxes to 220 - 240. Why? Because I would have a heart attack if I didn't.......

Is there any tonal value in this - probably not, many of the pre-war iconic guitars are fuzzy inside with tool marks and all manner of dirt and grime of the ages and they still sound great.

I usually go to 180 grit. I've also started using a brush coat of shellac on the inside
of the last couple. I intended to travel a lot with them, and I figured it would help
mitigate rapid RH changes. So far, so good.

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Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

Hesh, once again you have provided a great insight with a bit of humor thrown into the mix. Bill, I think the answer is simply that you finish out the inside to whatever level of finish and perfection suits your personal taste.....within reasonable parameters of cleanliness and artistry, of course. You will have to decide when it looks and feels good to you. When it looks and feels good to you, then you will have reached a reasonable parameter of cleanliness and artistry. As Hesh pointed out, it won't make much difference--unless you are building for a very picky client. Just my opinion, and I know others might disagree.
Patrick

I try to give the glue joints something to hold on to , so i only go to 120 because of them.

After the back and sides come out of the drum sander at 80 grit, i give them a super quick scraping, focusing on the parts you can easily see through the soundhole.

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Jordan Aceto
Ithaca, NY

For me, the soundboard goes to 220 grit. The back goes much higher - however, I use a razorblade to achieve the final surface but it is probably comparable to 1500 grit. The back bracing and kerfing goes to 320 grit. I use a razor blade to scrape the back after the bracing is in place and the back has been attached to the sides.

I have not gotten drawn into spending any more time inside the box than necessary. Tops and backs as they come off the thickness sander, braces to 120, with little concern for any squeeze out except on the part of the back that is clearly visible from the soundhole.

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David White, Toronto

"All my favourite singers can't sing."

Glue joints, with the possible exception of epoxy, are stronger when the surfaces are smoother. But 120 is smooth enough for most.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

Been going to 220, and using some maroon scotch brite too.

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John Mayes
http://www.mayesluthier.com

Same here, except I sand (roughly) with 180 instead of 220.

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

I keep all my glue joints at 120 no matter what glue for better holding power.Put some glue on glass and see what smooth does.

Actually Howard is quite right. The strength of a jointed edge is stronger than a rough surface. I thought we were gluing wood not glass? Although take some high strength hide glue and let it fully cure on glass and then try to get it off....yeah chipped glass.

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John Mayes
http://www.mayesluthier.com

Mark my friend if I recall another thread from the past here there were tests done on glue and joints and to make a long story short a freshly planed/scraped joint (15 minutes or less) and properly applied glue with the correct clamping pressure was the strongest bond. If I recall correctly Al C. mentioned this.

The inside of all of my major surface (back, sides and top) end up at 120 on the inside. Braces are sanded to 220.
A 120 grit finished surface is plenty smooth for any glue joint.

I loved looking inside all of the old Martins and Gibsons with their rough surfaces and deep tool marks in the wood.
While talking to Mike Longworth one day, he was telling me how the workers who installed the braces to the backs
would use a narrow scraper to actually make a couple of passes in the paths where the braces would lay before gluing
them down. The porous and absorbent characteristics of wood play a large role in the quality of any joint between
pieces of it. Even if you were to finish pieces at 400 grit, those pores and that absorption would still be chief
contributors to the quality and integrity of the glue joints.

I really can't remember a single brace being loose due to a bad joint on those vintage pieces. Any time we had to
reset and reglue a brace, it was because it had been knocked loose.

Martin has always finished their plates up at 120 grit as well. I had someone tell me that they spoke to a rep at Martin
and were told that they sand everything to 320. The funny thing is that no one on the floor who actually does the
sanding or brace installation has ever seen anything finer than 120 used for thickness sanding.

I'm not sure of what the other big shops finish up at, but I'd guess that it would fall close to 120 grit in most cases.

Regards,
Kevin Gallagher/Omega Guitars

Kevin,I always have used 120 for glueing as well.And i think it is the best grit for glueing a joint.I also like my joints tight.I was taught in wood shop along time ago not to starve the joint by clamping too tight or sanding too fine and i guess it has just stuck with me all these years.I have never had a joint failure.It has become a habit with me to glue this way.I also use TB glue for everything except my inlays,they are done with epoxy.And my shell is sanded to 120 when glueing also.I also have been in the Lapidary trade since childhood and wouldn't think of setting a stone in a setting with anything finer than 120.And i bet if you ask any dentist what grit his final bit leaves whendoing a crown, it will be close to the same,120.

I don't think anyone is arguing that 120 is not sufficient. You simply stated it was for better adhesion (paraphrasing) and that is incorrect. A joint sanded with 120 is fine. It's not as good as a planed one, but both work.....

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John Mayes
http://www.mayesluthier.com

John, I can see where scrapeing would leave the wood pores more open and would hold glue better as would a freshly planed surface,but i would not glue without sanding first.And i still feel that 120 is better when sanding.And that is why i stated that.

Of coarse, my back and top joints are of a planed finish as are the bottom surfaces of both my top
and back braces as they come off of the shaper and the jointer. I agree that a planed surface works well
for gluing wood pieces together, but I've never had a joint fail in either case so for me to decide which
better or worse just isn't possible.

Good is good and success is success. I do think, though, that the surfaces are affected by the presence
of the glues we use when joining wood pieces to one another. Wood absorbs the glue and there is always
some amount of softening and swelling at the joint site so I believe that the difference between a planed
joint and a 120 grit joint is minimized by those effects as well.

The 120 grit condition only comes into play when things are glued down to the back, top and sides so
it's sort of a combination of surface finishes being applied to one another.

I find it funny when someone does a test and draws conclusions as to what's best for glue joints. My
problem with it is this....Who's to say what the proper clamp pressure is? If there is the same clamp pressure
being applied in each variable's case? Who determines what is proper glue application for different kinds of
glue? is the amount of glue and its application exactly the same in each variable's case?

We all have our preferred methods for clamping, gluing, glue amount and how we apply it who's to say
which method is best and will yield the best glue joints. It's all so subjective that it's impossible to draw
any results that can be counted as real or true when a single person is doing the research.

I've glued using freshly planed surfaces...and I've glued using planed surfaces that have sat for months. The
results were exactly the same and both joints are still tight and together today. I've glued using 120 grit, 220
grit and scraped surfaces and all of those joints are still tight and together today. I can't honestly make a claim
that any are better than the others since they were all successful. As long as the surfaces prepared cleanly and
accurately, I'd say that all of the above can provide great joints that will last a very long time without failure.

Just my opinion,
Kevin Gallagher/Omega Guitars

First off let me say that sanding with 120 is probably adequate and will work fine etc. Most of the time one will rip wood off the opposing side of a joint when testing even after "roughing" with 120. That is the old "glue joint stronger than the wood" platitude. I would even go so far to say that it would be better to freshly sand with 120 than glue a old oxidized surface that was prepared more smoothly. The objection that has been raised was that it was stated or alluded to that it was desirable for some roughness for the glue to grab onto. This is a old woodworking myth. You don't need or want to give wood "tooth" as it is called. Folks need to have the right information and make decisions based on that. You don't want to think that if 120 is good because it gives some grip then 80 is better. Beginners might make that assumption or take what they read and repeat and spread a misconception.
With most of the common woodworking glues we use wood glues better when one surface is approximate to the other. (epoxy being a exception) This has been tested and proven.


Who's to say ? Well it is not just a single person for one. It is also not subjective. Most of these things are well known, proven and categorical. The "who" part of it would be wood technologists, scientists, and woodworkers. People with decrees in chemistry, engineering, doing extensive research and testing. People trying to keep planes in the air, people (companies) trying to avoid the litigious public. This is not a cavil argument. It is good to know that "tooth" is a myth and wood needs to be approximate. Of course there are other factors as it is easier to approximate softwoods than hardwoods because you can clamp or press the pieces closer or more proximal because it is softer. This means more clamping pressure with hardwoods than softwoods. This is proven but is easy to know intuitively as well. Surface area is a factor as well. If you have less than optimal surfaces but a large surface area the glue may be more than adequate. Load or how the joint will be stressed is a factor. All this needs to be weighed or considered. Sanding need to be done the right way as well or you will lose the accuracy of the joint independent of the roughness.
Knowing the right information helps when considering these factors or different situations. Knowing the right information helps to advance the craft and isn't that what this is about. In our shops we can know what is optimal and still make decisions empirically. To be clear I am not arguing that 120 grit isn't adequate, or that the way some folks do it won't work I am just railing against the old myth of "tooth.
Link

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Cut to size.....Beat to fit.....Paint to match.

FWIW I sand to 180 because I think it looks good and am too lazy to continue up to 320 or 400.

Regarding adhesion, lately I've been blowing all of my plates clean with compressed air before gluing braces. My thinking is that by removing all the sanding dust that gets packed into the pores of the wood (and there is a lot of it), I'm creating something closer to a scraped joint than just a plain sanded joint. Does anyone else do this? Should I bother with this step ... comments?




Bob K

Bob,
It is easy to do and can't hurt unless you have oil in your compressed air. Anything that gets between the two pieces of wood will not help your cause. In any woodworking joinery cleanliness is the way to go.
Link

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Cut to size.....Beat to fit.....Paint to match.

Bob,
I do the same. Bob Benedetto mentions doing this also in his DVD/book. I think it's a good habit to get into. Clean joints are happy joints!
dave

I go right up to 12000 micro mesh because I want a mirror finish inside the body

OK, just kidding.

I go to 220 just because. Use the ROS before gluing on the braces, takes a minute and makes it nice and clean looking (seems that's the new standard these days).

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My Facebook Guitar Page

"There's really no wrong way, as long as the results are what's desired." Charles Fox

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Good idea about making sure that you're not spraying oil. Hadn't thought about that, but it's the same setup I use for spraying nitro so I'm oil free.

Thanks for the affirmations and it's good to know I have something in common with Benedetto beside the same first name.