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Hello all

I got a client who came in the other day for repairs of a soundboard crack.

It so happen that he had another crack on the soundboard that was fixed by another luthier. The splint that was installed was noticeably lighter in colour than the soundboard (the guitar was built about 8 years ago), so he then took his guitar to another luthier who specialized in restoration. That luthier used a UV light on the repair to get it to darken to the surrounding colour.

I had heard and read about this technique, and was wondering if anybody here uses it. I would be interested to know, in particular, about:

-the type of light required (and where to get one)
-how close do you put it to the desired area, and how long do you let it "cook"
-how do you protect the soundboard around the area that you want to darken
-any other info on this technique that you could share.

Many thanks,

Pat

Pat,
Years ago I had though about this for furniture repair. I thought those uv lights the Dentist uses to kick off the acrylic in your teeth would work cool for this. I was thinking of using a mask of cardboard that had a hole cut in it that you hold off of the work surface similar to what you would do to feather a paint job. I heard those lights were very pricey though.
Link

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Cut to size.....Beat to fit.....Paint to match.

Hey Link

My ex-wife was an esthetician and she used an UV light for some sort of skin treatment. I recall it wasn't that expensive (a couple of hundreds?) but I am just not sure if it had enough power to do the job. That is why I am seeking advice, so that someone may know the necessary power is for the job.

Pat

Maybe a little harder to control, but there is an unlimited amount of free, high powered,UV light outside.

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Expectation is the source of all misery; comparison the thief of joy.
http://redrivercanoe.ca/

I used to use a blacklight box (thanks Lance McCullom!) to speed up the dry time of lacquer. It also emits a UV light and darkened the tops quite a bit. Don't know if that heps, but might give ya something else to consider.

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John Mayes
http://www.mayesluthier.com

A black light works fine for this. It usually takes a few days to a week under the light to get the desired effect.

There was a good article about UV light in the recent Violin Society 'VSA Papers' publication. The take-away message was that all of the effects produced by UV on wood were due to the prodution of ozone by the UV, which reacts with the nitrogen in the air to form nitric acid. This is basically (if you'll pardon the pun) a harmful reaction that breaks down the wood. There seems to be no way to effectively neutralize the acid once it's there.

Except possibly for some special reactions, it appeared as though anything we'd want to do with UV would be accomplished eventually by the lower-energy (and hence, safer) UV-A light that you can get from common 'black lights'. The author did not cover the use of UV to cure polyester, which may require a specific wave length to work most efficiently for all I know.

I have used black light bulbs to hurry along the drying of oil varnish. Some woods, particularly tropical hardwoods, contain enough non-drying oils to slow or nearly stop the drying of some oil varnsishes. Last spring, for example, I had problems with 'Rockhard' varnish not drying on some parts of the Macassar ebony binding on a guitar. Since I know from experience that you won't succeed with this stuff if the first coat is not fully cured, I used a small blacklight CFL and a foil lined reflector to help it along. The process took a day or two, and after that things went along as planned. Home made 'Fulton' varnish simply will not dry at all without strong UV light. Here in New England we don't get as much of that as reliably as Fulton does in the California Sierras, so a UV cabinet is a requirement here. In these cases I believe that the effects are due either to the energy of the UV photons absorbed by the varnish, or they might be from the ozone reacting directly to oxydize the monomers. Anyway, it works.

BTW, 'Rockhard' varnish flouresces under UV. So do Koa wood, and Black Locust.

The best tip I've heard of for matching the color of a splice is simply to cook the wood a little before making the splice. Of course, you can't do that once the splice is in.

Hey guys

That blacklight tip is quite interesting. I told the client that I may have the time to fix his guitar in May or June (presently pretty busy with a couple of orders and making 2 demo guitars for the Montreal Guitar Show), and since his Spruce top has gotten quite dark over the years, I figure I may need the UV treatment when I fix that particular guitar, and maybe others down the line.

Would you guys care to recommend any particular types of blacklight? I have never bought one before (although I do remember crusing the bars under blacklights back in the disco days of the late 70's and early 80's ). If you just have any pointers, including retailers, I would be very much obliged!

Cheers

Pat

I build fiddles and violins. I use UV "grow lights" from Walmart to darken wood and speed up drying time for oil varnish. Cheap and effective. You could mask off areas that you wanted to treat.

If you’re looking to set up a moderate to larger UV booth most pond supply stores or online shops sell 8.5" long 8watt to 50" long 65watt high intesity UV bulbs. Be carful these are very intense and can damage your eyes quickly

Thanks for the replies guys!

I will look into it.

Cheers

Pat

Is there a way to access the paper online? I may be missing something, but the nitric acid thing sounds a little fishy. UV light pops bonds on things and oxidizes them, but making nitric acid directly from nitrogen and oxygen gas wasn't something I thought was possible without extreme temperatures and/or pressures.

The UV lights they use for curing finishes are very specific wavelengths. I forget which, exactly, and I've heard there is some debate on which exact frequency works best.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Bob Garrish asked:
"Is there a way to access the paper online? I may be missing something, but the nitric acid thing sounds a little fishy. "

I looked in all the usual places, and can't find the issue: I may have loaned it to a student, so I'm working from memory. Again, the sequence was UV+3(O2)->2(O3)+N2-> 2(NO)+2(O2) and the nitric acid follows from there. It may take much less pressure and lower temps to produce NO with ozone, which is far more reactive than O2.

"The UV lights they use for curing finishes are very specific wavelengths."

I'm sure that's the case for the UV cure polyesters. The energy required to kick the reaction is probably known, and the coresponding wave length easy to calculate. In the case of a common varnish I'd imagine things are a little messier, since the chemistry is probably not as nicely controlled.

If the article is not on the VSA website I'd imaging you would need to join to get it, or else look up a memeber who didn't loan the issue out.

If they're going with that sort of sequence, then they took a leap they weren't allowed to (they only balanced the atoms, not the heats in the reactions). Elemental nitrogen (N2) is extremely stable and there are essentially no reactions with anything at standard temperature and pressure. To get it to spontaneously react with oxygen, ozone or not, they'd need to pop a triple bond and that requires insane energy (>2000 degrees Celsius!). I guess we're back with oxidation.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

That wasn't their reaction sequence, it was mine. I know that energy is needed to make it go. I also know that a photon of UV carries quite a lot of energy, and things like surface effects can alter the way reactions run. You do have a catalytic converter on your car, n'est ce pas?