Removal of poly finish ?

Construction and repair of Classical Guitar and related instruments
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Jedaks
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Re: Removal of poly finish ?

Post by Jedaks » Sun Aug 16, 2015 6:00 am

Pardon me for intruding here, but what is the acoustic difference between thin poly finish and the thin transparent plastic "guards" (golpedors?) that some high-end luthier built guitars have around the soundhole area?

Look at some good pics of professional quality guitars and you will see the thin plastic protective sheets.
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Re: Removal of poly finish ?

Post by simonm » Sun Aug 16, 2015 10:56 am

Jedaks wrote:the thin transparent plastic "guards" (golpedors?) that some high-end luthier built guitars have around the soundhole area?.
The guards are additional protection for the top for flamenco players. Flamenco players often hit the top of the guitar with their fingers for percussion effects. Their nails would eventually destroy the finish in that area. When there is a golpedor on the guitar it is intended as a flamenco guitar not a classical although obviously you can play whatever you want on it. You see the same thing on steel string acoustic guitars but the guard is for protecting the top against over enthusiastic use of a pick rather than fingernails.

Peterson2

Re: Removal of poly finish ?

Post by Peterson2 » Sun Aug 16, 2015 4:33 pm

rounie wrote:Hello Delcampers,
Controversial topic, but some of the well known luthiers i know use Nitro or a combi finish, my Howell is in fact 100% Nitro lacquered due to humidity and heat issues upon request to the luthier...sound amazing- Lately i have tried a guitar that had lacquer back and sides and oiled top which did sound good. Would be best to take a luthier's advice on this
you dont need a luthier's advice, you can use your own ears! your howell sounds amazing, i am sure it does to you, but you would be more amazed if you actually heard the difference of your guitar with different finishes and if were allowed to compare, ie. it could actually sound more amazing!
most makers thickness the guitar - but do not adjust for what finish is put on - so a guitar is never custom made to take nitro, or poly, or acryllic, or shellac. but the final finish makes a big difference to the voice, especially with how the voice develops as the guitar opens up, and how the guitar breathes during the year with humidity change.

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Re: Removal of poly finish ?

Post by Tom Sommerville » Sun Aug 16, 2015 11:06 pm

Hi Peterson2,

It sounds like you're talking about a polyester based, not polyurethane finish.

You could pitch a polyester coated instrument in a cage of gorillas and retrieve it a day later without so much as a scratch on it.
But it damps tone more than anything else.

I'm not sure about polyurethane.

It's hard to beat nitro, but an old nitro finish corrodes tuners and becomes flamable.

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Re: Removal of poly finish ?

Post by rounie » Wed Aug 19, 2015 3:45 am

Hello Peterson,

Point noted :) but the heat & humidity and relative lack of luthiers here in India combined with copious amount of sweating on my part

and fragility of FP do not permit me to entertain this finish in the forseeable future.

I have tried an oil finish classical by the local luthier and it was very very good...tempted to buy a guitar with this finish.
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Manuel Najera
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Re: Removal of poly finish ?

Post by Manuel Najera » Wed Sep 09, 2015 6:53 pm

Peterson2 wrote:get rid of the Poly rubbish, it really does contrain the sound, it is thick and usually many thick layers. with the poly removed, the guitar can finally breath! the wood can then vibrate naturally (like with a thin finish like french polish), and its voice can finally aslo start to develop. you will notice a massive difference in sound after 6 months to 1 year of playing after the poly finish is removed, even if it is a 30 year old guitar (that means the guitar has been choked for 30 years). I havent found shortcuts with any chemical cut, thinners or whatever, just get yourself a sharp paint scraper (and round the edges), and scrape off the finish, it takes ages -many hours, and you will need to resharpen the scraper blade, but this way also you can see when you get through the layers, and you wont guage into the wood beneath the finish. just be patient and think of the end result - a better sounding guitar. removing a poly finish off a classical guitar basically ends up in you hearing a totally new instrument - poly finishes are dreadful for sound, especially for a vibrant instrument like a classical guitar. poly finishes do their job. -- ie finishing, but they are not suited to classical guitar, they choke and constrain the sound too much.
Hello, I am new in this forum. But I have played guitar for 15 years and built them for 4 years (therefore, I am a beginner luthier) I also have a BS Degree on Mechanical Engineering.
Now, in my opinion, there are a lot of myths regarding the use of modern finishes (nitro, polyurethane, polyester, etc.). First of all, why would we want the guitar to “breath”?, is not this capillary property the first cause of cracks on hardwoods?
2) Neither the shellac nor other finish lets the guitar to breath (this is a fact, since all of them get in to the pores of the wood and seal them).
3) It is also truth that factory guitars are not French polished for one reason (time consuming = $$$$). And it is also known that they do not have too much care on how much material they add (one of the reasons of the “bad” reputation the modern finishes have).
4) Why would the back and sides need to vibrate? The vibration on the back means energy produced by the soundboard that is being wasted on the back. This is another myth. We want the back and sides to be as rigid as possible, why then we use hardwoods for it in the first place.
5) Newer and thinner finishes are being developed with all the benefits a finish should have for an instrument (hard to wear, thin, gloss, protection). Why would we get stuck with older finishes.

My point is, that no matters what finish we use (shellac, nitro, whatever), the most important thing is how much we use. I have played very good guitars that are French polished and that are polyurethane finished. They all sound great, and I do not think they can be improved by using a different finish.
That is my opinion, based on my years of experience playing and building. I would appreciate your comments on this.

By the way, I use polyurethane on back, sides and neck (wear areas) and shellac on the soundboard with very good results. But I am willing to try different finishes on the soundboard, since I am still learning this beautiful profession and there are a lot of myths that I want to get rid of.

One thing I read somewhere “A bad guitar with shellac, will still sound as a bad guitar”.

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Marko Räsänen
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Re: Removal of poly finish ?

Post by Marko Räsänen » Wed Sep 09, 2015 7:02 pm

Hello Najera_Luthier, and welcome to Delcamp forum!

Could I invite you to post an introduction of yourself in here?
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Re: Removal of poly finish ?

Post by hermit » Wed Sep 09, 2015 7:28 pm

My son has a guitar that looks like it was 'hand dipped' into a thick vat of poly. I read one reference to heat taking the stuff off but I haven't tried that. I figured I'd just start sanding with a course grit until I saw a little bare wood and start backing off after that.

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Re: Removal of poly finish ?

Post by kefroeschner » Thu Sep 10, 2015 8:55 pm

Nice discussion. I heartily agree that the thickness of the finish is important .. crucial perhaps, and that most 'factory' models have way too much. As to letting the wood 'breathe,' that is an unfortunately inappropriate metaphor -- what is wanted is to allow the wood to vibrate. Any finish put on bare wood will dampen it to some degree. In my shop i can tap an assembled top, whether spruce, cedar or 100 year old redwood and it will ring like a bell. Then just one coat of almost anything and it is noticeably dull. Things like the polyurethanes, acrylics and other modern finishes turn it to a material with the acoustic properties of cardboard.

The critical thing about any finish is its Modulus of Elasticity. This is a physics term for what is percievable as 'hardness.' Glass is hard, plastic is soft. Toughness may be desireable for protection but it invariably means low modulus and bad acoustics. Lacquer is hard, brittle and easily cracked, but these characteristics make it superior, acoustically to 'tougher' finishes. Shellac is basically the same as lacquer but without hardeners.

The secret to a superior, acoustically, finish with either (French Polish in the case of shellac) is keeping the film really thin. Most of what you put on in one coat must be sanded or scraped away, leaving only what fills the voids and pores behind. It will usually take many coats (most of which disappear) to fill the holes up to the surface. Then the final coat only needs to be really thin. It takes a lot of time and effort to do this and this, I think, is why most guitars have way too much finish on them -- you can also get to a final smooth top gloss surface simply by spraying on layer after layer and relying on surface tension to finally get to a smooth surface.

Bottom line -- high modulus, i,e brittle, not tough, finishing material and as little of it as possible.
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Chris Sobel
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Re: Removal of poly finish ?

Post by Chris Sobel » Fri Sep 11, 2015 12:37 am

Najera_Luthier wrote:
Peterson2 wrote:get rid of the Poly rubbish, it really does contrain the sound, it is thick and usually many thick layers. with the poly removed, the guitar can finally breath! the wood can then vibrate naturally (like with a thin finish like french polish), and its voice can finally aslo start to develop. you will notice a massive difference in sound after 6 months to 1 year of playing after the poly finish is removed, even if it is a 30 year old guitar (that means the guitar has been choked for 30 years). I havent found shortcuts with any chemical cut, thinners or whatever, just get yourself a sharp paint scraper (and round the edges), and scrape off the finish, it takes ages -many hours, and you will need to resharpen the scraper blade, but this way also you can see when you get through the layers, and you wont guage into the wood beneath the finish. just be patient and think of the end result - a better sounding guitar. removing a poly finish off a classical guitar basically ends up in you hearing a totally new instrument - poly finishes are dreadful for sound, especially for a vibrant instrument like a classical guitar. poly finishes do their job. -- ie finishing, but they are not suited to classical guitar, they choke and constrain the sound too much.
Hello, I am new in this forum. But I have played guitar for 15 years and built them for 4 years (therefore, I am a beginner luthier) I also have a BS Degree on Mechanical Engineering.
Now, in my opinion, there are a lot of myths regarding the use of modern finishes (nitro, polyurethane, polyester, etc.). First of all, why would we want the guitar to “breath”?, is not this capillary property the first cause of cracks on hardwoods?
2) Neither the shellac nor other finish lets the guitar to breath (this is a fact, since all of them get in to the pores of the wood and seal them).
3) It is also truth that factory guitars are not French polished for one reason (time consuming = $$$$). And it is also known that they do not have too much care on how much material they add (one of the reasons of the “bad” reputation the modern finishes have).
4) Why would the back and sides need to vibrate? The vibration on the back means energy produced by the soundboard that is being wasted on the back. This is another myth. We want the back and sides to be as rigid as possible, why then we use hardwoods for it in the first place.
5) Newer and thinner finishes are being developed with all the benefits a finish should have for an instrument (hard to wear, thin, gloss, protection). Why would we get stuck with older finishes.

My point is, that no matters what finish we use (shellac, nitro, whatever), the most important thing is how much we use. I have played very good guitars that are French polished and that are polyurethane finished. They all sound great, and I do not think they can be improved by using a different finish.
That is my opinion, based on my years of experience playing and building. I would appreciate your comments on this.

By the way, I use polyurethane on back, sides and neck (wear areas) and shellac on the soundboard with very good results. But I am willing to try different finishes on the soundboard, since I am still learning this beautiful profession and there are a lot of myths that I want to get rid of.

One thing I read somewhere “A bad guitar with shellac, will still sound as a bad guitar”.
Why would you want the guitar to breathe? So the wood oxidizes with time.

4) Why would the back and sides need to vibrate? A myth?

Most luthiers have built guitars with live or semi-live backs for the last hundreds of years, and for good reason. It produces a more nuanced tone that is overall richer in colors, and allows you to leave the top a little thicker since you are now involving a good sized resonant peak where the 'live' back is tuned. Just because you lose energy to the back is not necessarily a bad thing, depending on your building objectives.

Chris
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Manuel Najera
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Re: Removal of poly finish ?

Post by Manuel Najera » Fri Sep 11, 2015 4:48 pm

Peterson2 wrote:
Why would you want the guitar to breathe? So the wood oxidizes with time.

4) Why would the back and sides need to vibrate? A myth?

Most luthiers have built guitars with live or semi-live backs for the last hundreds of years, and for good reason. It produces a more nuanced tone that is overall richer in colors, and allows you to leave the top a little thicker since you are now involving a good sized resonant peak where the 'live' back is tuned. Just because you lose energy to the back is not necessarily a bad thing, depending on your building objectives.

Chris
Chris,

Like I said, I consider myself a beginner luthier, so please take my opinions as questions. And I really appreciate your comments.
Now, I am the type of person that needs to prove what everybody has been doing, even for years, just to understand why have they had been doing it like that, why does it work, and if it could be improved.

Regarding the "breathing" of the wood, isn't oxidizing a process of degradation?, if so, is it really a good thing for a guitar?
About the vibration of the wood, it make sense what you explained. Now, going back to whether poly is good or not for the back and sides (even on the entire guitar) I think it will depend on how it is applied (method, thickness, nature of the poly i.e. high content of solids). Otherwise, why there are very good guitars with non-shellac finishes?
How much would they improve in sound (if any) by finishing them with shellac? Is it worth it? And I am talking about great sounding luthier guitars. As I think we all agree that factory guitar finishes are, by far, poorly applied.

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Re: Removal of poly finish ?

Post by MarkInLA » Sun Sep 13, 2015 11:17 pm

THANKS FOR ALL THE ATTENTION to my OP. After reading them I think it's safe to say that the removal of the poly finish is not worth it, as result will be negligible. And the result could also be detrimental if not replaced with another finish. And the new finish could be very difficult to apply correctly, and, again, not worth it. In conclusion I think I should thus leave it alone... Mark

MarkInLA
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Re: Removal of poly finish ?

Post by MarkInLA » Sun Sep 13, 2015 11:21 pm

THANKS FOR ALL THE ATTENTION to my OP. After reading them I think it's safe to say that the removal of the poly finish is not worth it, as result will be negligible. And the result could also be detrimental if not replaced with another finish. And the new finish could be very difficult to apply correctly, and, again, not worth it. In conclusion I think I should thus leave it alone... Mark

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Manuel Najera
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Re: Removal of poly finish ?

Post by Manuel Najera » Mon Sep 14, 2015 3:15 pm

MarkInLA wrote:THANKS FOR ALL THE ATTENTION to my OP. After reading them I think it's safe to say that the removal of the poly finish is not worth it, as result will be negligible. And the result could also be detrimental if not replaced with another finish. And the new finish could be very difficult to apply correctly, and, again, not worth it. In conclusion I think I should thus leave it alone... Mark
Hi Mark,

I would not say that it is definitely not worth it. I did a little research on the Cordoba C10, and it looks like a nice guitar. In my opinion, why would I wanted to refinish a guitar:

1. The finish is already in a very bad shape
2. The factory finish it is noticeably too thick and the guitar is worth to try. In this case, I think it may be. I mean, you have a cedar top and Indian Rosewood back and sides, nice woods!.
3. I want to compare sound properties between finishes (a little unfair comparing to a factory guitar finish, but there is nothing to lose)

Now, if your guitar sounds already good, the final result of removing poly and trying a different finish may not be worth it. I do not know for sure.
If I going to do it myself on my free time, I would definitely do it. I think it would be a great experience and you could learn a lot from it (and share with us, of course :lol: )
If I am going to pay someone to do it, then I am not that sure.

But, if the guitar does not sound good at all, I think the finish is not the only one to blame. It could be a very thick soundboard, a very heavy bridge, a very stiff bracing, poor quality woods, etc.

I hope this helps!
:bye:

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Re: Removal of poly finish ?

Post by Catire » Mon Sep 14, 2015 9:47 pm

MarkInLA wrote:Hi. Not saying I'm going to, but if one did, how would the entire polyurethane finish be removed ? I'm guessing that chemical is a no-go as it could would affect the glues in all the visible areas; bridge, binding, purfling, rosette. If so, then I assume it's going to be sanded off. Yes ? I have this idea to refinish my Cordoba C10 with an organic substance; lacquer, varnish, (is it ?) French polish, other. I realize the poly finish may preserve the instrument for a longer time, or that it just goes on so evenly and dries quickly. But I keep wondering if the 'plastic' coat might be choking the wood's pores; that it would sound even better than it already does with a different coating. At the same time I am aware that the poly is very very thin and thus unlikely has any affect on the sound. Dibbs ?... Thanks, Mark
As you said, the poly is very thin. There's no point in replacing it with anything else. Taking the finish off a guitar, without damaging it is quite a skill; refinishing is another. You are going to botch your first one, trust me. Refinish a couple of beaters before you attempt anything serious. And no, the thin poly is not chocking your guitar anymore than any other type of finish.
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