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#1
06-02-2022, 07:12 AM
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Bee propolis and tung oil as acoustic finishes
I've been using tung oil for finishing electric guitars with extremely happy results, I can work multiple layers of dye and solid color pigments in, which become bound in the oil layers for color results that are unique (below)
I aim for many applications on the body (the only way to approach gloss with tung oil), and a single, or at most 2 layers on the neck, where I prefer near direct contact with wood & find matte finish offers less friction than gloss. On the new / rainbow in quilt maple, however, I would like to see a gloss finish on the body to best show off the maple chatoyance, and had a go at over-coating tung with bee propolis. That comes out gloss, and can be sanded out easily with 800-1200 mesh paper, I expect a polishing step after will go well. We keep bees and so I can use our own propolis, dissolved in pure ethanol. So here's my question: why not on an acoustic? The supposed downsides of tung (or boiled linseed) on a soundboard are that it never hardens, and that it will sink deeper into spruce or cedar, and dampen the top response. I can debunk the first positively. Fully polymerized tung is extremely hard, and if anything, runs a little brittle (with the advantage that spot fixes for minor scratches are extremely easy). On the second, it may be that tung will sink deeper into a soundboard, I don't know the viscosity of sprayed nitro, however in normal practice, the first layers of tung are thinned to make them sink deeper, so thinning could be skipped (or, I suppose, a single layer of nitro could be sprayed as a barrier) On the positive side, at least in it's liquid form tung is far less dense than nitro, which has a density ~1.3 g/ml. I'd think of this as an advantage in minimizing soundboard mass or maximizing stiffness:mass ratio. Tung is hard to polymerize in anything but extremely thin layers, on the other hand, I have access to equipment where I could measure it. Propolis, on the other hand is essentially tree sap. Bees collect it to coat the hive interior, where it provides the colony multiple health benefits, in addition to sealing cracks against weather. I don't think it's as protective as tung, though it's similarly hard. In any case, when I finally get to building an acoustic, I'm not yet decided on the need for a full gloss finish. The sound of the top wood especially is the main point of course. I'm also considering whether a mix of propolis with other natural varnish materials, like shellac would be a good choice. Last edited by Sadie-f; 06-02-2022 at 07:19 AM. Reason: Corrected some diction, clarified a couple of points 
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#2
06-02-2022, 08:36 AM
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I have built 3 acoustic guitars finished with Tru Oil, I assume it would soak in similar to tung oil. All 3 are excellent sounding guitars, I don't buy the tone dampening theory.
I think it also has an acceptable amount of gloss. [img]https://live.staticflickr.com/65535//[IMG] https://flic.kr/p/2nhRUap If you are still worried about soaking in, put a sealer coat of shellac on before your oil finish. Last edited by Fathand; 06-02-2022 at 08:42 AM.
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#3
06-02-2022, 08:45 AM
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Thanks for the feedback on tone!
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#4
06-07-2022, 10:22 AM
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I beg to differ. I think Tru-Oil is different from Tung oil. Tru-Oil is made from linseed oil and is more like an oil varnish. It does not soak very far into wood. It will build a film on the surface of the wood if enough coats are applied. I have never seen a Tung oil finish that had any measurable thickness on top of the wood surface.
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#5
06-07-2022, 12:55 PM
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#6
06-07-2022, 01:02 PM
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According to Ken Parker's Archtoppery youtube videos he also finishes his guitars with Tru Oil to get a polymerized film, but he seals the guitars with epoxy first, almost like the seal coat in violin finishing.
I've finished plenty of furniture with tung oil and love it for that purpose, but I think any kind of oil finish like that, even if it doesn't appreciably dampen the soundboard response (and I'm not convinced that it wouldn't, to be honest. It's not a theory, exactly, it's what oil does, though just how much it would dampen it is the question), would do very, very little to protect the wood, which seems like the other purpose of the finish on a guitar. Anyway, I would think for a gloss finish you could do like Parker does and seal the wood with a thin shellac or epoxy coat and then use Tru Oil to get the finish look you want; propolis and shellac also sounds like it might be promising. (I went down a violin finish rabbit hole a while ago. I didn't end anywhere, but I did learn that violin makers are like alchemists about their finishes, so I imagine if you looked around or asked in that sort of forum you might get pretty knowledgable answers about propolis and shellac, though probably not tung or Tru-Oil, as no one used those in the 1600s)
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#7
06-08-2022, 04:24 PM
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I'm not wild about delving into the violin rabbit hole, however I did and I guess I'm convinced of a couple of things. Violin makers say that finish can make the difference between improving with age and wearing out with age. I figure they also might have enough lore for that having basis. I expect there's more than one way to go about it. At least one note mentioned a varnish finish that's not tightly bound to the wood, I can see a sealer helping to accomplish this, and that a deep penetrating finish (oil or whatever) would bind enough wood fibers to each other to have an effect. That said, I disagree on your assertion that tung doesn't provide good protection. If that were true, it would also be true of Tru oil -- tung and linseed just aren't that different in my experience. What I find with tung, is after a month or so it becomes extremely hard (coincidentally, nitro needs a similar long time to fully harden). Once that hardness is achieved, it's main downside, is it's brittle enough to flake off when scratched. However, something that's going to do that, is going to damage any polymer finish. Where I love it, is that those small blemishes are super easy to fix: apply a single layer of tung, that will be cured in 24 hours, and additional work might be wanted if you want it to be invisible under close inspection. I've also made blemishes in nitro invisible - it's do-able, but much harder to accomplish. I have other reasons for avoiding Tru-oil and while they're extremely similar, I prefer tung to boiled linseed, mostly I'm familiar with it, I'm not saying tung is categorically better. I have years before it's going to matter, so I can do research / testing along the way.
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#8
06-08-2022, 09:39 PM
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You know, there’s no reason why you couldn’t finish an acoustic guitar with any of the above - each has pros & cons, and if the pros outweigh the cons for you, then do it! The only other thing you need to really think about, and is purely subjective, is whether a customer will feel your choice of finish adds value, or lowers cost, enough for them to buy your guitar over another. UV cured finishes tremendously lower cost - for many that’s a huge advantage. Some feel that french polish is the ideal finish - others like varnish. Both are far more labor intensive than UV cured polyester, but there are customers who will pay for that. Try it! It’d be fun to see and try one -
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