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(originally posted incorrectly in off-topic discussions Sorry!)

Hi everybody,

I have a spec guitar that I have been having problems with. The back and sides are cocobolo and in the bending process I got a bit of scorching in the waist. I then made the mistake of not fully sanding it off until I was doing the final detailing of the guitar. I thought I had pretty much completely removed it when prepping for and scraping the bindings but when I did my final sand that area lightened considerably and was now unoxidized next to the oxidized sides on either side of it.

Even at that point I was not worried, I expected it to catch up pretty quickly and even out. I felt like I had experienced that happening before successfully. I started the finishing process and thought it was moving slowly in the right direction but after a few weeks of the finish curing I realized it wasn't really going to happen. I stripped the finish off and have been blasting it with light a few days a week for over a month now and it is SLOWLY improving but at a total snails pace. It could be another 2-3 months it seems at this speed.

Does anyone have any suggestions of what I could do or what I shouldn't try? I have thought about just grinding away at the sides and exposing all unoxidized wood but after months of the rest of the sides darkening I am worried about how much I might have to take off. I can just wait of course but would rather not. I have thought about using a solvent to try to draw oils out but I am not sure that would really help anything.

Thanks for any help you can offer.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

I think I'd just wait. I also think you can go ahead and finish it. I don't think the finish will impede the ozidation. I don't know how it looks, but if it stays uneven it's just personality. I haven't worked with Coco due to allergies, but I had a similar problem with a Paduak guitar a while back. It still had a little contrast when I delivered it. It's owner didn't mind, and was actually a little dissappointed when it evened out.

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

On a recent build I milled up some cocobolo binding and each consecutive slice was lighter and lighter. After a couple of hours outside on a sunny day they all matched. YMMV.

Bob

Hey Burton,

I recently had a similar issue with some cocobolo. It darkened after I bent it in some areas I then wiped it with a lacquer thinner. This evened all of the color up and seemed to lighten it some. This may have not been oxidation and merely just resin coming from the rosewood, but its worth a shot. Make sure to keep rotating your rag until color stops staining your rag.

Rob

Burton, do you have a small iron to heat up the area? Maybe that would help. I usually see my coco bindings darken a lot when bending. I have also used the lacquer trick on other rosewoods

Just a thought.

Ken

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http://www.casperguitar.com

My experience is in contrast to Woody's: I've found that finish definitely slows down oxidation.

How about a sunlamp? I've used them but be careful to not look at it very long.

I believe it is UV light that causes wood to darken with exposure, It's not really an oxidation. The change happens under finish as well. It's just that the finish may slow the process depending on how much UV it reflects. The light source you are using may be really low on UV output, try a UV light source like a black light bulb and see if that doesn't do the trick

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

I think darkening due to UV exposure vs oxidation depends on the species. IE, Spruce darkens with UV exposure, but Paduak oxidizes. I'm not sure which happens with Cocobolo. Perhaps it's a combination of both on most species. Maybe John Arnold will read this and respond. I have no doubt he'll have the correct answer.

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

My understanding is - and I'm going on memory as I haven't looked at wood chemistry in a long long time - that it's an oxidation process that's greatly accelerated by exposure to UV light. Even in a dark corner the wood will slowly change color as it oxidizes, but the lignin in wood absorbs UV light which adds energy to the system to speed things along. Heat also accelerates the process. There are probably chemicals in the extractives of cocobolo that speed things up as well since it changes color really fast compared to most woods.

To the OP, time and diffuse sunlight is probably your best friend.

Oxidation is not what really happens that causes the darkening we see. Oxidation is that brown, dusty film that occurs on the surface of lumber during storage. It must be removed to finish the wood or it will for a barrier in the finish and prevent the finish from adhering. Oxidation cannot occur without oxygen, once the wood is sealed and can no longer react with oxygen, no oxidation can take place. What we see is a color shift in the wood itself brought about by a photo-chemical reaction within the wood. Certain species show a more pronounced effect than others. Cherry is a good example of a highly photo-reactive wood. Spruce hardly shows much reaction at all. Some woods actually lighten with exposure, I have seen walnut do this. Some wood changes color entirely like Purpleheart.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

I recently resawed some bocote for a build. The board was a rich very deep brown with nice black highlights. Of course once cut, the deep browns were hugely lighter, almost yellowish. I have been wondering about a way to deepen it more quickly. Seems a similar issue in this discussion.

What about potassium dichromate? Haven't heard it mentioned in a long time. Does anyone have experience/knowledge about it?

Here is the result of a google search:
(https://www.google.com/#sclient=psy-ab&hl=en&source=hp&q=potassium+dichromate+wood&pbx=1&oq=potassium+dichromate&aq=3&aqi=g4&aql=&gs_sm=c&gs_upl=0l0l1l3459l0l0l0l0l0l0l0l0ll0l0&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=4c3fe1a9859f48e4&biw=900&bih=747)

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Chuck Kish

I've used potassium dichromate on mahogany furniture projects. The effects are instant and dramatic. It only works with woods that have tannins... I do not know the tannin content of cocobolo's extractives so I'm not sure if it works with it or not. I can tell you it is highly toxic so extreme caution needs to be taken when handling it.

Another potassium compound that works is potassium permanganate , usually available at pool supply places. Test it on scrap first.

Thanks very much for all the replies.

That is interesting distinguishing between oxidation and photochemical changes, I hadn't thought about the difference. I'll check the bulb I am using to see if it is UV high or low (i hope low!). I am very nervous about using any chemicals and probably will not pursue those. I will try a few more light regimens though and see how it goes. Since I am not in a super rush I think I will wait on the finish a bit longer. I was going to use the EM7000 which I believe has a UV blocker in it. That might make me wait a bit longer still. I wish I could leave it in the sun but with it being winter I don't have a good way to do that and still keep it humidified. Maybe a balck light is the trick. If I do that I might have to inlay some sunglasses into the guitar somewhere.

Thanks again guys,

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Burton
http://www.legeytinstruments.com
Brookline, MA.

With some woods heat will bring the oils out and effect a color change. I have seen this happen with purple heart. I don't know if this would work with cocobola, but if you have a scrap of wood and a heat gun you could do a quick test. Obviously there is some risk when heating thin woods with a heat gun, so user discretion is advised.
I have also used detergent to remove surface oils from cocobola . This seems to keep it from turning dark as quickly as it seems to do otherwise. It does raise the grain a bit, and I lightly sand back before finishing. Again testing on scrap to see if this would work for you is advised.