Removal of poly finish ?

Construction and repair of Classical Guitar and related instruments
MarkInLA
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Removal of poly finish ?

Post by MarkInLA » Mon Aug 10, 2015 1:00 am

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

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bear
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Re: Removal of poly finish ?

Post by bear » Mon Aug 10, 2015 1:18 am

I've had to strip a couple of cg's. Use the finest grade steel wool you can get. 0000
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Philosopherguy
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Re: Removal of poly finish ?

Post by Philosopherguy » Mon Aug 10, 2015 1:44 am

I imagine just refinishing the top would accomplish what you may want. Taking ALL the finish off would be a long and likely unnecessary process.

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mmapag
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Re: Removal of poly finish ?

Post by mmapag » Mon Aug 10, 2015 2:38 am

The difference between poly and FP would be marginal if discernible at all. One of the benefits of poly is that is can be applied more thinly than Nitro. The effort would be substantial and the return low. I've refinished nitro using chemical strippers in the majority of the body and sanding with 400 grit along the edges and the rosette with no ill effects. Just a messy, time consuming job.

In my case, the instrument was a little beat up and the finish was thick nitro. The result sound wise and appearance wise was worth the effort.
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MessyTendon
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Re: Removal of poly finish ?

Post by MessyTendon » Mon Aug 10, 2015 4:47 am

I have only french polished one guitar and the results were quite good. I had an old harmony guitar which had nitro lacquer that was crazing and the finish was a rather ugly vintage red brown...I can't say it improved the sound because I sold the guitar, but the buyer was totally blown away. The new finish really looked awesome.

Use chemical stripper and mask the bridge off, this is the main area you should do most of the hand sanding. The chemical will strip the finish, and you will want to use a plastic scraper...one wrong move with a metal scraper and the guitar will get dinged quite bad. I would suggest using the lodge brand cast iron scraper, they are a hard polycarbonate scraper.

You could sand the finish but that would take forever and the nasty fine particles would get everywhere. The chemical stripper is nasty but it leaves a gooey mess instead of a fine dust.

I am recommending you do this project only if you want to learn to appreciate the labor intensive process of french polishing. I do believe the neck does not need to be finished with shellac, you could simply use an oil rub finish on the neck, that is up to you.

Use shellac flakes and grain alcohol. Here in California we can't get the higher proof required to dissolve the shellac, but in other states you can get high proof grain alcohol. This makes for a relatively safe nontoxic finish.

It will take a few weeks to build up a really nice finish.

MarkInLA
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Re: Removal of poly finish ?

Post by MarkInLA » Mon Aug 10, 2015 8:03 am

Wow ! Thanks for all the positive replies. I was all set to get warned to not go there; that it's a can of worms. Philosopherguy, I admit I was thinking of doing the entire box and might have if you didn't say that. Makes sense. It probably would lower the value if I sold it, as anyone looking for this model would probably pass if it weren't original. And really, in the end I think the results would be minimal on the sound quality side, if at all. That poly is really thin on this guitar...It's way on the back burner anyway....At least I know now that it's not an uncommon procedure...

Keith
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Re: Removal of poly finish ?

Post by Keith » Mon Aug 10, 2015 9:03 am

a couple of notes: 1. removing finish increases the possibility of taking off wood which may seem to be a positive thing with a tank like the c-10 except the loss of wood is likely to be unequal across the area--a little too much wood taken here, less wood taken off there. 2. finishing is an art form and if one does not have that art form down pat the results may be less than desirable. I would bet the folks who French polish guitars had to go through a few trials to get it right. nitro requires a booth, machines, and of course, technique.

I refinished a cordoba 50r guitar neck and used off the self polyurethane which worked well. folks have used tru-oil with positive results and it is easy to apply. I reshaped the neck which was my intent but the stripping was messy and some wood did come off during the stripping process. I can only imagine doing the entire guitar would be nasty.
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Michael.N.
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Re: Removal of poly finish ?

Post by Michael.N. » Mon Aug 10, 2015 9:31 am

You could end up lowering the value by a significant margin unless you are good at polishing around bridges! It's hard enough for folk who have a lot of experience with French polishing.
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OldPotter
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Re: Removal of poly finish ?

Post by OldPotter » Mon Aug 10, 2015 2:43 pm

Use chemical stripper and mask the bridge off, this is the main area you should do most of the hand sanding. The chemical will strip the finish, and you will want to use a plastic scraper...one wrong move with a metal scraper and the guitar will get dinged quite bad. I would suggest using the lodge brand cast iron scraper, they are a hard polycarbonate scraper.
I have tried using chemical strippers on polyurethane, I found that the stuff we can get over here barely had any effect. Bearing in mind that we are not allowed to get the old fashioned strong stuff unless we are a professional company. Then the cost of shipping hazardous chemicals comes into play.
I ended up using scrapers and sandpaper, I wouldn't want to do this again.
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Peterson2

Re: Removal of poly finish ?

Post by Peterson2 » Mon Aug 10, 2015 4:38 pm

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.

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Re: Removal of poly finish ?

Post by simonm » Mon Aug 10, 2015 8:40 pm

Philosopherguy wrote:I imagine just refinishing the top would accomplish what you may want. (…)
In fact there are some professions who finish the body with nitro (or other finish) and french polish just the top. There are a number of reason for this. The top is the bit that benefits most from a really light shellac finish. The back and sides get way more wear and tear rubbing constantly against the players body and a finish such as nitro is harder wearing. In a workshop where many guitars are being finished then this method is more economical in time. Only the top gets the most timing consuming finish. In some ways this mixed finish is the best of both worlds.
So long as you finished the top well, I would argue that it is value enhancing rather than taking value away but that would depend on the perception of the buyer.

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Chris Sobel
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Re: Removal of poly finish ?

Post by Chris Sobel » Tue Aug 11, 2015 2:27 am

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
I recommend investing in a good luthier built guitar that is french polished. It will not keep you up at night wondering if something is getting 'choked' on the guitar! :)

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Moje
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Re: Removal of poly finish ?

Post by Moje » Tue Aug 11, 2015 8:50 am

The urethane isn't choking the sound, any audible benefit will be purely imaginary. A refinish on a C10 couldn't possibly be worth the money or effort.
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Peterson2

Re: Removal of poly finish ?

Post by Peterson2 » Tue Aug 11, 2015 4:00 pm

Moje wrote:The urethane isn't choking the sound, any audible benefit will be purely imaginary. A refinish on a C10 couldn't possibly be worth the money or effort.
from my refinishes i have done, yes it is choking the sound, and often HUGELY. if you do it yourself it is worth the effort.

rounie
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Re: Removal of poly finish ?

Post by rounie » Tue Aug 11, 2015 5:07 pm

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
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