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Finish - how thin is too thin?
https://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=42954		 Page 1 of 1
Author:  tjp [ Wed Mar 12, 2014 3:53 pm ]
Post subject:  Finish - how thin is too thin?
Or, when to say when... I flat sanded my latest, #5, which is finished using a wipe-on Ace varnish/Tung Oil/Mineral Spirits blend (which I like a ton).

Each of the ten coats consisted of wiping on a thick coat, waiting a bit, then wiping excess off.

After sanding, it was pretty darn flat, no witness lines, good coverage, so I decided to buff it to a nice semi-gloss finish to see how that went. Still looks pretty darn good to me, though I know it's thinnish...def not the dipped in plastic look we sometimes see.

Do you all have a rule of thumb for when to stop? I keep looking at it, and the little devil on my shoulder says "aw c'mon, I know it looks good, but wouldn't it be better after one more coat??"

Finishing is a black art, I'm sure of it. :)
Author:  Fred Tellier [ Wed Mar 12, 2014 4:23 pm ]
Post subject:  Re: Finish - how thin is too thin?
If you have not sanded or buffed through then it is not too thin.

Fred
Author:  Joe Beaver [ Wed Mar 12, 2014 4:29 pm ]
Post subject:  Re: Finish - how thin is too thin?
I'm with Fred on this. Especially with a tung oil finish. It stands up pretty well and receives 'repair' coats pretty well down the road.

Me? I'm more of a dipped in plastic guy but do sometimes use an oil finished neck.
Author:  meddlingfool [ Wed Mar 12, 2014 6:10 pm ]
Post subject:  Re: Finish - how thin is too thin?
Is that a high gloss finish?
Author:  Tom West [ Wed Mar 12, 2014 7:04 pm ]
Post subject:  Re: Finish - how thin is too thin?
Fred Tellier wrote:
If you have not sanded or buffed through then it is not too thin.

Fred

I agree. Extra finish is weight. Weight is a response and tone killer in my mind. This is why you see some folks use lacquer on all but the top which they French Polish. True Oil, polymerized tung oil, etc. all fit in this category. A thin finish is a good thing..........!!!
Tom
Author:  tjp [ Wed Mar 12, 2014 7:37 pm ]
Post subject:  Re: Finish - how thin is too thin?
Thanks guys, those were my instincts... I think it "could" be a high gloss finish but I'm not good enough to pull that off to my satisfaction (yet). The varnish I use is gloss, same as Bruce S uses, but I'm not sure what effect, if any, the tung oil has.
Author:  B. Howard [ Thu Mar 13, 2014 7:19 am ]
Post subject:  Re: Finish - how thin is too thin?
In my experience a finish much under 3 mils of dry film weight will not survive.
Author:  Alan Carruth [ Thu Mar 13, 2014 12:20 pm ]
Post subject:  Re: Finish - how thin is too thin?
It depends on the finish.

Plain drying oil finishes don't really form a protective film, so there's no 'thickness' to speak of. They do soak in and add mass. Oils also have high damping.

Some of the film forming finishes are 'tough' and others are 'hard'. Most of the oil-resin varnishes, for example, are on the 'tough' side: they remain flexible, and dent rather than scratching. Nitro is a 'hard' finish. UV cure poly seems to be both hard and tough.

A thin film of a hard finish will protect a soft surface, like a cedar top, better than the same thickness of something tough. On a hard surface the tougher finish may give better protection, since it will remain unbroken even if it's dinged a little.

There's also chemical resistance to think about. Shellac is actually reasonably hard, and quite tough, but it's usually used thin (as French polish), and does not resist common solvents (including sweat) as well as many other finishes do. Some oil-rssin varnishes also react to perspiration, as do some of the earlier water based finishes I tried (which can really put you off a repeat of the trial!).
Author:  B. Howard [ Fri Mar 14, 2014 7:29 am ]
Post subject:  Re: Finish - how thin is too thin?
The varnish and tung oil listed as ingredients both form a finish film.....
Author:  Alan Carruth [ Fri Mar 14, 2014 12:25 pm ]
Post subject:  Re: Finish - how thin is too thin?
To clarify: I was talking about plain 'boiled linseed oil' and the like. Some things, like Tru-Oil, if I've got it right, are polymerized oils, and may form a film. There's a lot of new chemistry and technology out there that I'm not familiar with.

That said: in his study of finishes Schleske found that any finish containing oil tended to have higher damping than a 'resin' finish like shellac or lacquer. OIl-resin varnishes tends to add damping commensurate with the proportion of oil they contained; with 'rubbing' varnishes adding less than 'spar' varnishes. Any oil that has low molecular weight components will also tend to soak in more, and thus add more mass, than a 'resin' finish.

I've used oils once in a while, but am not a big fan of them.
Author:  JustinNorth [ Fri Mar 14, 2014 9:21 pm ]
Post subject:  Re: Finish - how thin is too thin?
Not that this really matters, but on the newly redesigned Taylor 800 series they wanted to get to a final thickness of 3 mils and couldn't get it to hold up at that thickness. They've gone with a 3.5 mil final thickness on the finish.

This supports what B. Howard was saying earlier about anything in the 3 mil or less range not being thick enough.


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Author:  John Arnold [ Fri Mar 14, 2014 9:52 pm ]
Post subject:  Re: Finish - how thin is too thin?
Quote:
on the newly redesigned Taylor 800 series they wanted to get to a final thickness of 3 mils and couldn't get it to hold up at that thickness.

The lacquer on the top of the guitars I make is in the 2 to 2.5 mil range after buffing. I am not sure what is meant by 'hold up', since the durability of a thinly-applied lacquer or shellac finish is a function of how it is treated. IMHO, if I am not buffing through the finish occasionally, it is too thick.
Taylor uses polyester, not lacquer, and I have seen adhesion problems with those types of finishes. With lacquer, adhesion should never be a problem even at 0.5 mils, provided the surface is compatible (not too oily).
Author:  tjp [ Fri Mar 14, 2014 11:44 pm ]
Post subject:  Re: Finish - how thin is too thin?
Good discussion guys, glad it's still alive. FWIW, I sealed with shellac before a couple of coats, wet sanded to pore fill, then the 8 or ten coats I spoke of. Gonna give it a shot. Goes to my bro-in-law, so he knows where to find me if it doesn't work, and I get to monitor it a couple times a year. I love the oil/varnish look and feel. Two questions left:

1) The shellac ought to prevent oil penetration, shouldn't it?
2) Is the degree of dampening inherent to oil finished significant? I have a hard time believing that it is, but I am willing to be educated.
Author:  Michael.N. [ Sat Mar 15, 2014 4:38 am ]
Post subject:  Re: Finish - how thin is too thin?
I know of quite a few makers using an Oil finish/Varnish directly applied to wood. They tend to be makers of Lutes and other very low tension stringed instruments but there are also a few Classical Guitar makers in the mix as well. Any dampening must be so tiny as to be insignificant. If you can't hear it on a Lute (and these people can't, neither can I) when will you hear it?
Author:  Alan Carruth [ Sat Mar 15, 2014 6:40 pm ]
Post subject:  Re: Finish - how thin is too thin?
As Michael says, its 'significant' if you can hear it. With my hearing I'm not the one to say. The problem IMO is that if you do hear it, it's too late: there's no way to get that stuff out of the wood once it's in.
Author:  JustinNorth [ Sat Mar 15, 2014 8:29 pm ]
Post subject:  Re: Finish - how thin is too thin?
John Arnold wrote:
Quote:
on the newly redesigned Taylor 800 series they wanted to get to a final thickness of 3 mils and couldn't get it to hold up at that thickness.

The lacquer on the top of the guitars I make is in the 2 to 2.5 mil range after buffing. I am not sure what is meant by 'hold up', since the durability of a thinly-applied lacquer or shellac finish is a function of how it is treated. IMHO, if I am not buffing through the finish occasionally, it is too thick.
Taylor uses polyester, not lacquer, and I have seen adhesion problems with those types of finishes. With lacquer, adhesion should never be a problem even at 0.5 mils, provided the surface is compatible (not too oily).


Maybe the description I heard from the retailer I talked to was wrong. Maybe Taylor didn't want to admit that they couldn't find a way to go thinner in their mass manufacturing setup. They're really trying hard to convince people of the new 800 series being more "hand built".


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Author:  John Arnold [ Sun Mar 16, 2014 2:39 am ]
Post subject:  Re: Finish - how thin is too thin?
Quote:
1) The shellac ought to prevent oil penetration, shouldn't it?

Yes.
Quote:
2) Is the degree of dampening inherent to oil finished significant? I have a hard time believing that it is, but I am willing to be educated.

I believe it is. The buzzword these days is a 'varnish' finish (whatever that means). All I hear in the instruments finished with that as opposed to lacquer is a lessening of the high frequencies. Of couse, this effect is magnified when the finish gets thicker.
I have always thought that the best sounding guitar is one with no finish on the top. It's not very practical, however.
Author:  B. Howard [ Sun Mar 16, 2014 8:45 am ]
Post subject:  Re: Finish - how thin is too thin?
John Arnold wrote:
The lacquer on the top of the guitars I make is in the 2 to 2.5 mil range after buffing. I am not sure what is meant by 'hold up', since the durability of a thinly-applied lacquer or shellac finish is a function of how it is treated. IMHO, if I am not buffing through the finish occasionally, it is too thick.



I have been involved in more than a few field assessments of lacquer cabinet finishes and resultant film failures over the years where actual film thickness was measured with an Eddy current device. These inquiries and tests were always conducted hand in hand with a field chemist from my coatings manufacturer. At thicknesses below 3 mils the finish becomes more prone to chipping and flaking in areas of normal contact, like around pulls or work areas where people stand and rub against the finish while working. This has absolutely nothing to do with finish care but simply with chemistry and film integrity. Wood moves...a lot, and has special needs in regards to flexibility and expandability in a coating. Lacquer cannot reliably meet those requirements long term at thicknesses that are to thin to be functional. When a coatings engineer tells me they consider 3 mils to be a minimum for that type of coating and will not warrant coatings applied thinner than that......well that told me all I needed to know. After all they have spent a lot of time and money developing a line of coatings and application guidelines for the successful use of those products and one will never go wrong following their guidelines.

Other Coatings have different barriers. For instance a clear conversion varnish will be quite happy at around 2 mils. It has a film barrier at the other side where exceeding 5 mils of total film thickness will cause crazing, flaking and de-laminations. Pure urethanes will also be quite happy at very low mil application and some may have maximum film thicknesses as low as 3 mils. I have never used a UV cure polyester so I can't comment on Taylor's findings but being that polyesters of one source or another are the heart of most modern acrylic coatings I would not be surprised that about 3 mils would be a minimum with those as well.
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