Official Luthiers Forum!

Hello all together,
why do some luthiers finish their guitars with oil? Has this type of finish tonal quality advantages? Or is it only because of that it is a lot easier to apply, timesaving, no need for french polishing or laquering skills?

Some luthiers use oil varnish as a finish. This is especially popular with violin family instruments where several coats of varnish can cut down on the screechy-ness of an unvarnished instrument. I have an old guitar made by a violin maker that has back and sides varnished, but they left the soundboard bare. Some modern makers use oil varnishes on all the surfaces of their guitars. It is not a particularly easy finish to apply.
I have used a light coat of a drying oil to enhance the color of the wood on the back and sides of some instruments, and after letting it dry a couple of weeks top coated it with lacquer. That worked O.K..
When I first started building instruments in the 1970's I finished a couple with "Danish oil" (Watco) but think that it killed the sound some.
Some people use "Tru oil", which is a wipe on oil/varnish and is similar in application to French polishing shellac. It doesn't seem to "sink in" to the wood like boiled linseed oil and works O.K.
There are a lot of things called "oil finish", many of which are not strictly oils.

so what is "tru oil"?

Here's a link: https://www.birchwoodcasey.com/products/tru-oil-stock-finish-3-fl-oz-liquid.html

_________________
Steve Smith
"Music is what feelings sound like"

Tru oil is a "Var oil" according to a finisher friend of mine , a mixture of varnish and linseed oil. The SDS lists the ingredients as linseed oil, proprietary modified oils, and Stoddard solvent (naphtha). Varnishes are oils modified by cooking them with a resin to form a new compound. My guess is the linseed oil and naphtha is added to make the varnish component into a wipeable finish (speculation on my part). The SDS lists Tru oil as a mixture, so some of the ingredients do not chemically combine.

Getting an oil varnish on thin and looking good is just as difficult as any other finish, possibly more difficult. Nitrocellulose lacquer is maybe the easiest choice but there is still a learning curve and no shortcuts. Most finishes, applied very thin, will not adversely effect the sound of a guitar. Most finishes, applied thick, will adversely effect the sound.

Drying oils add a lot of damping, which tends to 'eat' high frequencies. A drying oil also does not form a permanently waterproof film: once the oil has cured it's no longer oil, and is not waterproof. To preserve the water resistance you have to keep adding oil. That's not a problem on a gun stock, but on a guitar it soaks in, adding mass and, of course, more damping. Finally, some oils, such as linseed oil, really never stop curing, hardening and shrinking. This can eventually cause problems in itself. Making an oil-resin varnish stabilizes it, limiting the shrinkage, and the resin produces a water proof film.

So, where does Tru-Oil fit into this? Is it water proof?

Oil finishes are used by some on guitars because it's thought to be easier than spraying traditional nitro lacquer. French polished shellac is also used by some for the same reasons while others use it because it is likely the most transparent finish in terms of tone that is commonly used for guitars. This is why the classical guitar community where responsiveness has to be optimized likes French polished shellac.

I see lots of small builder instruments with oil finishes and although it can be made to look pretty nice and certainly nice enough for a small builder to use it does not look as good as some of the other traditional guitar finishes in my experience and opinion and I think it's less ding resistant too.

It's been a problem for all of time for a small builder to find an easy to use, tough, serviceable finish that is suitable for objects that expand and contract with the seasons.

I never got there but had I remained interested in building after my 54.5 builds I was migrating to a French polished shellac top with nitro back and sides.

Bruce Sexauer does an oil varnish and he was kind enough to send me his recipe and finish schedule. It was by far the most time consuming finish I have ever done. To make it look as good as possible takes a lot of work. Of course with experience that would speed up the time too, but still. I like the look of it though. It's not a mirror polish but has a very organic looking warmth to it.

Of all the finishes I have tried, which most except for modern UV stuff, good old classic nitro is the easiest though time consuming in that you have to wait to polish it out. I did one guitar in Tru-Oil and that was pretty easy to apply as well. The very first guitar I ever build going on 30 years ago now was a wipe on poly 'varnish' which was also pretty easy to apply.

I think anyone can be taught to French Polish even on their very first guitar and get reasonably descent results. You always hear that it's labor intensive, requires skills and so on, and it does like any finish to get real professional results, but a first timer or student can get very good results applying shellac with a pad.

guitarjtb asked:
"So, where does Tru-Oil fit into this? Is it water proof?"

I don't know. I've seen various posts in different places that give different recipes for Tru-Oil. Some say its a 'modified oil varnish', some a say it 's a mix of oil and oil-resin varnish, and some say it's a polymerized oil. Without some resin in there I'd doubt it could produce a film that would remain water proof for a long time. I know it's customary to re-oil gun stocks periodically, if only to patch up dings and scratches so they remain water proof, but I've never tested the stuff myself. I will say that 'water proof' is something of a relative term. The reason the Navy went to fiberglass small boats was that a study they did showed that even 1/4" of epoxy does not make a 'water proof' film for wood. Keep in mind that the Navy test for a 'water proof' glue is to stick together two 1" cubes of wood and then boil them for 24 hours. They're pretty serious about 'water proof' (as you might be if you've been above the arctic circle in a storm with 50' seas) ...

I know there's a few guys using Epiphanes High Gloss. I haven't tried it, but I believe it's a spar varnish and supposed to be waterproof.

Laurent Brondel for example.
He believe mixes Epifanes Clear varnish with the Epifanes Accelerator plus thinner., adding a little acetone to help with bonding the layers..
Makes for a very warm, amber gloss finish.
I recently finished my bathroom in a satin Yacht varnish, made me think about trying The Epifanes on a guitar.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

'Oil finish' and 'varnish' are different things; the conversation here seems to slide from one to the other without respecting that difference.

Drying oils, such as linseed and walnut oil, harden by an oxidation reaction, shrinking as it does. This can continue for a long time, and produces a film that is not water proof, but is quite flexible and chemically resistant. The lower molecular weight components of the oil can penetrate quite deeply into wood, adding weight, and drying oils also have high damping.

Many 'varnishes' used by violin makers are essentially mixtures of resin in a solvent, usually alcohol. Shellac is widely used this way, but there are a number of other resins. These harden by solvent evaporation, leaving the resin as a water proof film, which will be softened by any contact with the solvent. Most natural resins are fairly brittle: shellac is an exception because it is cross-linked by the bugs that produce it. This also makes the film progressively more solvent resistant.

Traditional oil-resin varnishes are made by cooking a drying oil together with a resin to produce a co-polymer. The resulting tarry goo is dissolved in a 'vehicle', such as turpentine, which makes it spreadable. When the vehicle evaporates the varnish hardens by oxidation of the oil component, which can proceed for months, although many varnishes will be mostly cured within hours. The co-polymer has a high molecular weight, so it doesn't soak into the wood. The oil confers flexibility, chemical resistance, and toughness to the film, and usually a refractive index closer to that of the wood than most resins. The resin content makes the film more water proof more chemically stable (it stops drying and shrinking, eventually), and harder.

'Varnish', like 'wood', is a word that covers a class of materials that can differ greatly from each other. Even if you confine yourself to 'natural' resins and oils there are a number of them, which can be combined in different proportions, cooked at different temperatures and for different lengths of time, all of which affect the final product. Some varnishes can last for centuries, and others break down fairly quickly, and the difference might simply be in how they were cooked. Meyers, in his 'Artist's Handbook of Materials and Methods' says that it's impossible to produce oil-resin varnish in batches of less than about 200 gallons with reasonable quality control. It's fun, and informative, to cook your own once, but in practice it's better to find a good commercial product.

Just to muddy things; you can mix an oil-resin varnish with a drying oil and some vehicle to make a wiping finish. I believe that's what at least some of the 'Danish oil' finishes are. I have no idea about how that stacks up as a finish for guitars.