Curing Finishes With UV

Danish oil will build. . . if it contains a resin. The mistake you are making is assuming that all Danish oils are the same. They are not. Some contain a resin, others are just a mix of drying oils. Anything that contains oil and a reasonable proportion of a resin will build. There's no one single definition of 'Danish oil'. In the broader sense it simply means a wipe on finish, as opposed to a brushed on finish, which tends to contain less solvent. The actual ingredients of a wipe on finish and a 'varnish' can be exactly the same.

I get that, Michael, but anything that is wipe on/wipe off will give a very poor build. It enriches the look of the wood but offers scant protection to the surface. Without renewing the finish at some point the project will look and feel like smooth raw wood with no luster.. I've found it difficult to sell instruments with such a finish.
 
What's the problem?

You know that is a good question. I've used tung oil on wood and the sound was fine. I just refer to the "collective wisdom" that oil will dampen the sound. I have no evidence that this is so nor have I encountered it because I don't use the stuff anymore. Maybe it is just a prejudice and I'm not going to say that "oil" dampens a soundboard because I'm not sure that is true. Shrug. It all becomes a subjective judgement and I suspect the effect is small.

I think the bottom line of this thread should be: Tung oil is a great finish for the hobbyist/first timer/casual builder to use and can be applied a number of ways including a rag and still get a good looking, good sounding finish with a minimum of experience, cost and work. It is pretty close to fool-proof. The only other negative I can think of is that once you lay down that tung oil, you are committed to using oil from that point on. You can't really decide that after putting on a tung oil finish decide you want to try shellac or nitro or...
 
There is no collective wisdom that "oil" dampens sound and this is emphatically true when "oil" is used loosely to include a wide range of finishes with disparate characteristics. Formby's and Tru-Oil are wiping varnishes that harden. Given the confusion out there that sellers of finishes often don't try to dispel, some people might miss the point that products that are marketed as "tung oil" don't necessarily have much to do with tung oil. Formby's and Tru-Oil are as different from tung oil as cats are different from cattails.

Just trying to keep my thread from going off on a wild ride on the wrong bus. :)
 
Last edited:
I have a tin of pure tung oil (that's the oil by itself, nothing else added). It would take dozens of coats to obtain any sort of build and likely the resulting finish would be on the soft side.
If you look at instructions for making your own 'Danish oil' or wiping varnish the usual advice is to buy an ordinary tin of oil varnish and thin it down with a solvent like turpentine. That can be the difference between an oil varnish and a Danish oil/wiping varnish - just the amount of solvent. The end product, when the solvent has evaporated, will be exactly the same. The difference is that the oil varnish is brushed on in a much thicker coat. The wipe on is thinner but tends to go on in a more even coat. The latter requires more applications.
Now we come on to the terminology of 'Danish oil' and other oil type finishes. It can mean virtually anything. Manufacturers use the terms as it suits them. You have to look at the small print to see whether the stuff is just a drying oil or a drying oil that also contains a resin. Don't take a blind bit of notice of the large letters on the front of the tin. Resins are hard, tend to be glossy and they build quickly. Oils are thin, flexible and relatively soft. It's fairly obvious what each component brings to the mix. In other words don't expect an oil only finish to build quickly, be very hard and very glossy. It isn't going to happen.
As for oil dampening sound. There have been tests done that show this, by Martin Schleske the violin maker. The question then becomes a matter of whether we can hear that difference. I'm not sure that we can but if you want to play it safe it's a simple matter of placing a sealer coat under the oil finish. That's what the violin makers do. I also do it on soundboards but not on the back/sides where I want the oil to pop the grain.
 
You know that is a good question. I've used tung oil on wood and the sound was fine. I just refer to the "collective wisdom" that oil will dampen the sound. I have no evidence that this is so nor have I encountered it because I don't use the stuff anymore. Maybe it is just a prejudice and I'm not going to say that "oil" dampens a soundboard because I'm not sure that is true. Shrug. It all becomes a subjective judgement and I suspect the effect is small.

I think the bottom line of this thread should be: Tung oil is a great finish for the hobbyist/first timer/casual builder to use and can be applied a number of ways including a rag and still get a good looking, good sounding finish with a minimum of experience, cost and work. It is pretty close to fool-proof. The only other negative I can think of is that once you lay down that tung oil, you are committed to using oil from that point on. You can't really decide that after putting on a tung oil finish decide you want to try shellac or nitro or...

Wrong. I've sprayed nitro over my Danish oil intruments after the oil has dried well and the wood has taken a desicated, raw look. No problems involved. I don't know how much earlier in the life of Danish oil I could have done this. I don't finish with shellac, but as oil is involved in getting a build without the tampon sticking to the surface I suspect the shellac might stick to an oil finish. But again, I don't know from experience.
 
I really wonder sometimes how much a "finish" has to do with how an instrument sounds. I suspect much less than we like to think. The sound has a lot more to do with the soundboard thickness and the bracing than anything we slather on the outside. There for I like to put on a pretty finish on the instrument and that means shiny, shiny and that means shellac.... what was the original thread? I think I forgot.

DSCN7554.jpg
 
Whatever the original excuse was for the thread, it's been great to give Michael and John a reason to pass on some of what they know about finishing. :)
 
First of all David, Congratulations on your "cure cabinet". Very nifty!

I've probably spent more years in finish work and formulation than anything else; there are a lot of good observations by Michael & John - I'll add a bit to those.

Yes, Danish Oil is pretty much useless as a specific description and "Tung Oil" is very seldom simply that. I've used the pure oil before; it will build, but the soft tendencies Michael spoke of will be a big problem without totally impractical dry times between coats.

As far as shellac sticking to an oil finish, depends on what formula and again, how long it has cured. Works better the other way around.

And so most important to this thread, is that I do believe there is a (slightly) discernible difference in soundboard response when most "oil" varnishes are applied directly to a soundboard; most especially softwood. I would recommend one coat of shellac first. When we used oil varnish (made up from our own resin mix) since we did shellac first for the soundboard; at that point, though it was likely unnecessary, it was no big deal to do it to the body as well.

The plus is that this yields a very flexible, yet still very protective coating, as violin makers always aspire to.
 
Good point on the use of shellac as the first coat, Dirk. I aways do that, usually well before the actual finishing phase. It helps to protect the top a bit and to keep it clean.
 
Wrong. I've sprayed nitro over my Danish oil intruments after the oil has dried well and the wood has taken a desicated, raw look. No problems involved. I don't know how much earlier in the life of Danish oil I could have done this. I don't finish with shellac, but as oil is involved in getting a build without the tampon sticking to the surface I suspect the shellac might stick to an oil finish. But again, I don't know from experience.

French polishing over oil varnish with shellac is exactly what a high end mandolin maker I know does. He says it's the traditional finish for the instrument and I saw Len Dudenbostl do the same in a a video on making mandolins.
 
Just over a week ago I dug out an old bottle of pine resin varnish that I made 7 or 8 years ago. I slapped some on a bit of ebony and placed it outdoors in direct sunlight. We had a decent few days of sunshine but it's been under cloudy weather since. I've just left it there day and night. For some reason this stuff is proving to be quite a bit harder than I remembered it. Most of these violin type varnishes tend to be a bit on the soft side for us plucked instrument players. Either my memory of it is wrong or it's somehow matured in the bottle. The smell of the varnish in incredible, worth using on that basis alone!
I vowed never to make the stuff ever again (acrid fumes and it's dangerous) but I'm almost tempted. . . .
 
Having now read this together with the Luthiers Forum post, I'm thinking that leaving Tru-oil to cure in the dark in a windowless workshop may not be such a good idea?
I wasn't aware of the influence of light on curing oil finishes - I just assumed solvent evaporation given a warmish dry environment would be enough. Given what I have learned here I doubt I have been leaving my instruments long enough to cure fully before re-coating or buffing out, which may explain apparent softening of the Tru-oil finish on the back of some necks. The idea of taking bits of instrument into the house to cure in daylight isn't going to go down very well with her ladyship though!
 
Tru Oil has an added chemical drier, which is why it does dry without much light. UV just speeds it up. Dry isn't necessarily fully cured though. Fully cured happens over a period of months (perhaps years) but of course diminishing returns (in terms of hardness) is going to set in just after it's touch dry. In other words it's not going to continually harden to the point that it becomes something like an extremely hard floor varnish!
Whether UV hastens this hardening process is open for debate. I, along with others, suspect that it does but I'm not sure we can state it as fact.
 
Last edited:
I think we have to define terms: There is "drying" and then there is "curing" of finish's. Drying is an evaporative process where a solvent (such as water or alcohol or whatever) evaporates and leaves behind a substance. Evaporation is a function of time and temperature and relative humidity. Curing however involves no evaporation but a chemical cross linking bond that is formed between molecules. Think epoxy or CA. And then there are finishes that are a combination of the two such as nitrocellulose which is a combination of the two. UV does not effect evaporation but it does hasten chemical cross linking and thus hardening of the finish. Therefore, an evaporative finish is not going to harden any faster with UV radiation.

However, modern finishes (I think) are a combination of evaporative finishes mixed with cross-linking finishes so they would "cure" with UV. I also think that UV is a quick fix but one could over do it and create a brittle or over cured finish. UV radiation is powerful stuff and can destroy as well as cure. Think sun burn.
 
Shellac is an example of an evaporative finish. Eventually it can become impervious to it's solvent, so presumably it can also cure.
I've yet to experience or hear of one single problem with UV curing. Any problems usually come with varnishes that have added driers where the film dries from the top down. Hard on top, soft underneath can be a recipe for cracking. Unlikely with something like Tru oil which is put on in extremely thin coats. The UV is similar to the UV that you experience on a sunny day but out of direct sunlight. In other words my varnish cures just as quick outdoors (maybe a bit faster) as it does in my UV cabinet.
 
Very early on, I tried that. I hadn't yet figured out that oil often doesn't cure on certain woods. I put Tru-Oil on a Bolivian rosewood tenor and when it remained gummy, I put it out in the sun. Too much sun. For too long. The back cracked. Lesson learned. I fixed it and it was a great player for a long time. Eventually gave it to a kid who played the heck out it.

That's why I put a fan in my garbage can to pull out the heat.
 
"The WORLD’S ONLY grow tent for attics, lofts and basements is now here."

Nice! I wonder how many of these could be sold here in the land of "grow your own"?
 
Top Bottom