Question about nitrocellulose finish

Construction and repair of Classical Guitar and related instruments
LBrandt
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Question about nitrocellulose finish

Post by LBrandt » Thu Oct 11, 2018 7:47 pm

I’ve never owned a guitar with a nitrocellulose finish, but I’m considering a guitar that has a nitro finish, and I have a few questions about such a finish.

I’ve read that a nitro finish never “cures”, and that it’s easily marred. Now, I’m not one that worries about nicks, etc., as I know that a guitar is to be played and not put on display. One or two of my polyurethane finish guitars has the occasional nick, and that’s ok.

My concern is that I’m just wondering about the durability of a nitrocellulose finish, so I’m asking those of you who have guitars with a nitro finish to advise me on the subject.

Thanks,

Louis

Wuuthrad
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Re: Question about nitrocellulose finish

Post by Wuuthrad » Thu Oct 11, 2018 9:27 pm

Nitro doesn't scratch as easily as French Polish or Shellac, but it does have a tendency to crack or check over time due to rapid changes in temperature and/or humidity.

Nitro checking is a sought after feature for many guitar players, so much so that you can pay a lot of money for a "weathered" or "antiqued" electric guitar.

Whether or not this is important to a classical guitarist is more a matter of opinion, and not as much a feature of the market.

Nitro is a more durable finish, and may constrict woods' vibration more, although it is not as easily repaired as Shellac. It is originally an automotive finish and has partially contributed to the high gloss feature of guitars.
I prefer it to a Poly finish, but it is environmentally unsafe.

I like the feel of it myself, as it's generally less sticky for my sweaty fingers than Shellac, which I personally cannot stand due to its fragility (as a former Violinist I absolutely prefer Oil Varnish or Tru-Oil,) and I do like the look of a cracked and weathered nitro!
"Pay no attention to what the critics say. A statue has never been erected in honor of a critic." -Jean Sibelius

Echi
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Re: Question about nitrocellulose finish

Post by Echi » Fri Oct 12, 2018 8:29 am

Nitro is in my favourite finish in terms of tone and is also quite good in terms of future repairability.

The main drawback here is that it’s very unhealthy and this makes it an inconvininient option for the luthier.
The second big drawback is that it is an unstable finish: here in Delcamp Alan Carruth wrote about this aspect quite exhaustively, better you to use the search mode and make reference to him.

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petermc61
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Re: Question about nitrocellulose finish

Post by petermc61 » Fri Oct 12, 2018 11:29 am

My experience is that nitro and suction cups are a disasterous combination.

Marcus Dominelli
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Re: Question about nitrocellulose finish

Post by Marcus Dominelli » Fri Oct 12, 2018 3:50 pm

I like nitro lacquer much more than polyurethane, but I still prefer french polish for its ease of repair.
Nitro will react to some plastics, vinyls, and rubbers, if they contact the finish for long.

The first guitar I built was a telecaster copy, which I sprayed with nitro lacquer. I had the guitar stored in a gig bag for a while, and I noticed that the red fluff which formed the nice soft padding inside the gig bag was reacting to the lacquer. It was breaking down the lacquer, and the lacquer was taking on a reddish color.
This kind of thing happens with nitro. It slowly degrades until it's full of cracks, like you see on those old Gibson guitars from the 50's. Lots of character, but french polish can be easily over coated and touched up, so you can keep the finish looking fresh, if not pristine.

Alan Carruth
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Re: Question about nitrocellulose finish

Post by Alan Carruth » Sat Oct 13, 2018 10:28 pm

Nitro is harder than shellac, so it is 'more durable' in the short term in that respect. In the long run nitro breaks down chemically: it's considered 'toxic' by museum curators because it releases nitrates as it does that ultimately become nitric acid on reaction with humidity and air. Nitro has to be isolated to protect the rest of the collection. Shellac starts out somewhat cross-linked, which makes it less likely to chip or crack than nitro, and shellac becomes more cross linked over time. 'New' shellac is soluble in alcohol and alkaline water solutions, which can include sweat. As it ages it becomes less so, and after 75 years it's insoluble in anything.

Finish chemists keep coming up with harder and harder finishes. Shellac used too be considered a 'hard' finish in comparison with oil varnishes. It was superseded by nitro. Now nitro is considered a 'soft' finish by comparison with UV cure polyester and some others.

Hard finishes add a lot of stiffness, particularly in the cross grain direction on the top. This has got to restrict the movement of the top. Generally speaking, sprayed finishes go on thicker than FP, which adds mass, and that doesn't help either.

If you treat a guitar right there is no reason for the finish to get dinged up. IMO the use of hard finishes on low-end guitars has lead to players not appreciating, or even knowing, the sort of care that a fine instrument should get. There are plenty of hundred year old French polished or varnished guitars out there that look fine because the owners treated them right. In the 'Dangerous Curves' show of guitars as art object several years ago most of the older guitars looked much better than the Martins and such from the 40s and 50s that had nitro finishes.

LBrandt
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Re: Question about nitrocellulose finish

Post by LBrandt » Sun Oct 14, 2018 1:01 am

Thanks for the information. I'm not a good enough guitarist to be able to tell the difference in sound between poly, french polish and nitrocellulose, so all of my guitars, even the more expensive ones, have a poly finish. But I've read that nitrocellulose never actually "cures", and that's why I've stayed away from it. Maybe the source in which I found that statement was incorrect.

Alan Carruth
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Re: Question about nitrocellulose finish

Post by Alan Carruth » Mon Oct 15, 2018 1:22 am

Nitro is a solvent release finish, like shellac. You mix the stuff up with the proper solvent and paint/spray/pad it onto the surface. The solvent evaporates and leaves the finish behind. It doesn't 'cure' in the sense that a varnish finish does, by a chemical reaction that changes the nature of the finish. Lacquer thinner is much more volatile than the alcohol that is used with shellac, so lacquer hardens faster. It can still take a few weeks or longer for all of the thinner to be released, but shellac is still slower.

BTW, I'm told that the least toxic thing in lacquer thinner is toluene. That's toxic at a concentration 1/10 of what you can smell. Wear a hazardous vapor mask with a nice fresh cartridge when you apply it. By comparison, if you use pure ethyl alcohol to dissolve shellac the thinner is non-toxic in moderate amounts. Shellac itself is, at worst, inert: you'd get a tummy ache if you ate a lot, but it's not poisonous. They do use it in food as a coating in small amounts, I'm told. The other thing that's used in French polish is oil, often olive oil.

Any finish primarily changes the tone of the guitar according to how much of it there is, and how stiff it is as a material. A very thin coat of almost any finish will have pretty much the same, minimal effect on the sound. One reason FP is so good is that it goes on thinner than most other finishes. Of course, a really thin coating of even a very hard material won't protect the top against dings and/or scratches, particularly if it's a soft wood like cedar. I've been using a very hard and tough oil-resin varnish of late that can be applied only a little bit thicker than FP. It gives more protection, without a noticeable penalty in sound, but it can still be dented easily just because it's so thin. We're talking about a thickness of about .002" (.05mm) for FP, and (with luck) .003" for this varnish. Manufacturers can put on sprayed lacquer finishes at around .005" if they really try to keep it thin; .007" and more is more common. The prize (?) goes to an Ovation guitar I re-topped a few decades back that had .04" (a full millimeter) of epoxy on the top for a finish. You could not scratch it, but the top could not move much to produce sound either. That's the trade-off.

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