Question about finish

Construction and repair of Classical Guitar and related instruments
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Steve Ganz
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Re: Question about finish

Post by Steve Ganz » Sat Jun 10, 2017 8:21 pm

Water based finishes can be OK.
Common complaints: difficult to touch up showing "contour lines", bluish tint or cloudy, plastic appearance.
Steve

JohnH*
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Re: Question about finish

Post by JohnH* » Wed Jun 14, 2017 4:11 pm

No one has yet mentioned that nitro cellulose lacquer is toxic to both the finisher and the environment.
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Jose Marques
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Re: Question about finish

Post by Jose Marques » Wed Jun 14, 2017 7:56 pm

JohnH* wrote:
Wed Jun 14, 2017 4:11 pm
No one has yet mentioned that nitro cellulose lacquer is toxic to both the finisher and the environment.

Yes you are rigth and poly as well exept the water based

The question about do a shelack finish mate is a good question
Oil finish is a good decision as well but must be very careful in the top
I'm a Luthier living in Bury st Edmunds UK

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Alexandru Marian
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Re: Question about finish

Post by Alexandru Marian » Thu Jun 15, 2017 10:21 am

The nitro solvents are nasty indeed but these days good protection is easily available and affordable. I think it might be more hazardous to French polish and soak up all that alcohol rather than spray with a good mask and ventilation. As for the environment, I don't drive a car but bike/subway so that balances things :)

Alan Carruth
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Re: Question about finish

Post by Alan Carruth » Thu Jun 15, 2017 5:26 pm

The testing I've seen shows nitro as a harder finish than shellac, but also more prone to chipping and scratching. Shellac is tougher because of the way it is cross linked; it's more likely to dent than scratch.

Many years ago I did a science project on plastic, and one of the ones I made was nitrocellulose (imagine!). You can nitrate cotton to various levels; at about 18% nitration it becomes soluble in a range of very nasty things like MEK. In use they limit the amount of these by cutting lacquer thinner with somewhat less toxic things. The least toxic ingredient in most lacquer thinners is toluene, and that's toxic at a concentration of 1/10th the level you can smell.

Highly nitrated cellulose is a high explosive: each molecule contains everything it needs to 'burn', and it only requires a bit of a kick to start it off. UV light can supply that. Even the low level nitrated stuff we use as a finish is 1/6 high explosive; not enough to actually blow up, but it does burn enthusiastically, and suffers from the same lack of stability in the long term. A student of mine who worked with museum conservators said that they rate a nitro finish as having a useful life of about 75 years. Given that a well made guitar can last for well over 100 years, why use a finish that will be flaking off long before that?

Several years ago the Boston Museum of Fine Arts assembled an exhibition of guitars as art objects. It was easy to see in that show when the Martin company switched over to nitro; the earlier guitars that had been finished with shellac and varnish looked much better. There was a Stronberg arch top in the show that had a tick nitro finish, along with celluloid (the same material) for trim. The bindings were cracked every inch or so, the headstock veneer looked as though it had been burned with a torch, and chicks in the thick finish were continuing into wood. It's no wonder they left that out of the catalog: there was no way to get an attractive photo of what was otherwise a fine looking instrument.

Varnish is not a panacea, of course (no finish is). 'Run' copal varnish turns black overt time, and rosin varnish shrinks and cracks badly.

If it were not for the initial issue of wear shellac would be the best of them all. Sadly, it's soluble in alkaline water solutions, which seems to include some folk's sweat. The solubility decreases over time, so that after about 75 years it's impervious to almost everything. It's hard to get the customers to wait that long. :(

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Alexandru Marian
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Re: Question about finish

Post by Alexandru Marian » Thu Jun 15, 2017 7:36 pm

Could it be that modern formulations are more stable? The stuff I use is mixed with both alkyd and acrylic resins. I also thinking that nitro flaking on old guitars can also a mechanical problem. Constant expansion and contraction of the wood will dislodge the lacquer off. It can flake right off if you spray it over too glossy wood, for example an ebony fretboard edge.

Either way, 75 years seems long enough to me. Shellac can last less than 1 year given the right player. Most every player that actually uses the guitar seriously rather than keeping it on the wall will burn through the shellac on the back and some side areas in a few months to a few years. And they usually come for refinish only after the wood itself is damaged (discolored by sweat, pores popped open, glue lines and purflings damaged etc etc)

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Re: Question about finish

Post by Alan Carruth » Mon Jun 19, 2017 5:02 pm

I find that oil-resin varnish is a decent solution. A good one will be harder and much tougher than shellac, but not as hard as nitro. Varnishes cn be quite stable chemically, and because they cure by a chemical reaction they are not soluble in whatever solvent/vehicle was used to make them spreadable, nor are they usually soluble in other common solvents. The lack of solubility can make them difficult to touch up if they do get damaged. They also tend to go on a bit thicker than shellac used as French polish, and have higher damping. Since we usually try to avoid that, especially on the top, I often FP the top and varnish the rest. It's hard to beat the transparency and depth of a good varnish finish; not even with FP.

altnaez
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Re: Question about finish

Post by altnaez » Mon Jun 19, 2017 7:21 pm

I have never built a classical, but I have built two electrics using water based laquer with great results. A few years ago I purchased a
1988 Greg Byers with FP cedar top with water based laquer on B&S. I think its the best combo for classicals. I have another two classicals, one with laquer B&S FP top, and another totaly FP. The laquer is chipping of the back of the neck on and is deteriorating were it has come in contact with ones skin. the FP one has the fabric from the case liner imprinted were it comes in contact.
I know that living in the tropic (warm weather & high humidity) plays an important role.
However the Byers B&S are like new, with only the usual wear on the FP top.
Saludos,
Albert

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Steve Ganz
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Re: Question about finish

Post by Steve Ganz » Mon Jun 19, 2017 7:26 pm

Al Carruth, Do you cook up your own oil-resin varnish or use one commercially available ?
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Paul Micheletti
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Re: Question about finish

Post by Paul Micheletti » Mon Jun 19, 2017 8:10 pm

I'm planning on using Royal Lac on my next guitar for back/sides/neck and FP for the top. I'm quite allergic to nitro fumes, so that is out of the question. And I've always hated the plastic look of urethanes and water based finished. Time to give a catalyzed shellac product a try.

Alan Carruth
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Re: Question about finish

Post by Alan Carruth » Tue Jun 20, 2017 6:03 pm

I did cook up one batch of varnish, a long time ago. It was fun, and turned out usable, but was 'way too much work. For the most part I've used commercially available ones, and that has it's own issues. The most aggravating is that every time I get used to a varnish to where I can get good, consistent results, it's discontinued. In one case it was because the manufacturer had trouble getting the resin they were using, but for the most part I suspect it has to do with solvents and ozone. The stuff I'm using now is Murdoch's 'Ure-Alkyd 500' floor finish, made by Sutherland-Welles. The produce the resin from the waste of the Cabot cheese plant, and the oil is, I believe, tung oil. They thin it out with a citrus solvent. I have had some problems getting it to harden reliably in some tropical woods, but when it works it's one of the best I've tried.

Another varnish that folks hve used on guitars for some time is Pratt & Lambert #38. I did try that on a test piece, and it looked good. Then I heard that P&L was being bought up by another company, and figured that they might well change the formula or discontinue #38. From what I understand that didn't happen.

If you're looking for a 'traditional' alternative to nitro, and don't want to FP for whatever reason, you should try one of these.

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Steve Ganz
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Re: Question about finish

Post by Steve Ganz » Wed Jun 21, 2017 7:28 am

Thank you Al. I've tried some oil varnish and the dry time seems ok, but the harden time ... no. I was aware of P&L but not Murdoch's. Is the time to harden acceptable ?
Steve

Jose Marques
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Re: Question about finish

Post by Jose Marques » Wed Jun 21, 2017 8:11 am

you can look for the http://nutshellpaints.co.uk/shop/tuf-wood-oil/ i have used one time only, because i continue toprefer shellack but is good, Andy Manson and Ergon Guitars use this with the wax ate the end with very good results

i have used the danish oil as well in the past and i like it
I'm a Luthier living in Bury st Edmunds UK

Alan Carruth
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Re: Question about finish

Post by Alan Carruth » Wed Jun 21, 2017 5:49 pm

I guess it depends on what you deem to be 'acceptable'.

Often I find that the first coat will be slow to cure fully, taking anywhere from a couple of days to a week depending on the wood. Once the first coat has set up properly I can often re-coat within 6-8 hours, and always within a day. Due to the nature of oil varnish it's wise to re-coat a soon as possible, or, at least, not to let it go too long between coats, as that invites 'witness line' problems. Don't try re-coating before the first coat has set up; it may never harden and the improper bond will allow the whole finish to peel up.

Also due to the nature of the finish it will be much slower to fully cure than most of the solvent release finishes, and it will shrink for as long as it's curing. I find it really helpful to allow the piece to sit for a month after applying the last coat, as this takes care of most of the shrinkage. Personally, I don't mind seeing the grain of the wood showing through, but that's not the level 'new Mercedes' look that the market values. If you don't mind the shrinkage it's possible to put on a finish inside of a week in most cases with this stuff, but I usually try to allow for a couple of months from start to finish. If you build the time into your schedule it's not a problem. Since most of it is spent just listening to the varnish dry you can do other things, so the main issue is the space the stuff takes up hanging there.

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Steve Ganz
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Re: Question about finish

Post by Steve Ganz » Wed Jun 21, 2017 7:33 pm

Thanks for that Al.
Steve

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