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Hairline Cracks in Macassar Ebony Back

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Hairline Cracks in Macassar Ebony Back

Postby Philip Secrist » Fri Feb 24, 2017 3:00 pm

Hey guys, I built a dreadnought guitar with a three piece back out of beautiful Macassar Ebony with sap wood. There are 3 long hairline cracks that have opened up in the sapwood area of the back. (See the attached pics). They showed up after I had sprayed the guitar and let it cure for a few weeks. I thought that they were just grain lines that had not been filled entirely with pore filler and that the nitro finish has just sunk in. I sanded and level them out but they reappeared again after a week or two, so I leveled and buffed again. I sanded and level them out and buffed it and then gave it the customer.

I got the guitar back after about 9 months for a checkup and lines have appeared again. After sanding with 1000 grit to see if I could level the finish, I can now see that they are cracks. I can't get the cracks to move at all when I push on them, so I am not sure they go all the way through the wood, but they definitely have opened up the finish and I can feel them with my finger.

Two Questions: 1) How do you recommend I repair these cracks? 2) I am not confident that I have enough finish there to level sand after the repair, since I leveled the area three times before thinking the finish was just shrinking into the depressions (dang it!). So, If I end up sanding down to the wood after the repair and need to re-spray the area to build up the finish and cover the repair, how do you recommend I do it so that I don't get over-spray on the rest of the guitar or leave lines where I mask the sides and top? Thanks for you help!
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Re: Hairline Cracks in Macassar Ebony Back

Postby David King » Fri Feb 24, 2017 3:13 pm

The wood looks closer to flatsawn which would make it less stable than quartersawn. That said you'll probably want to glue the cracks with cross-grain patches on the inside of the guitar. Once the wood can't move your finish will be more likely stay put on the outside. Do you think the wood is fully cured and you have a stable relative humidity in your work space? I'd be a little concerned about more cracks showing up later as the RH changes.
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Re: Hairline Cracks in Macassar Ebony Back

Postby Philip Secrist » Fri Feb 24, 2017 3:18 pm

Thanks David. I can't see the crack lines on the inside and the one at the top runs very close to the neck block, if not under it. Do you recommend super glue from the finish side to fill the cracks? I do keep my shop humidity between 40-50%.
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Re: Hairline Cracks in Macassar Ebony Back

Postby Barry Daniels » Fri Feb 24, 2017 7:11 pm

Superglue to fill and then cleats with HHG would be my approach. I level the superglue using Earlwine's razorblades method where clear tape is used on the blade ends to prevent over leveling.
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Re: Hairline Cracks in Macassar Ebony Back

Postby Philip Secrist » Fri Feb 24, 2017 11:26 pm

Correction, I can actually see the cracks on the inside of the guitar, so they do go all the way through. Should I feel with superglue from the inside and then cleat with hide glue? How do I minimize the glue going through to the finish side? They are pretty tight cracks, maybe that won't be a concern? Should I try to clamp over the crack slightly toopen the crack a little on the inside and close it on the outside?
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Re: Hairline Cracks in Macassar Ebony Back

Postby Ryan Mazzocco » Sat Feb 25, 2017 1:33 am

Fill with CA from the finish side. I'd say don't put any unneeded stress on it with clamps; the superglue will wick it's way down in there without any help from you. Fill the cracks with the super glue just like you're doing a drop fill. Let it dry completely, maybe even letting it sit overnight, then scrape with a taped razor blade like Barry said. Then cleat on the inside.
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Re: Hairline Cracks in Macassar Ebony Back

Postby Philip Secrist » Thu Mar 02, 2017 6:50 pm

Should I use thin or medium viscosity superglue. Will medium wick into the crack well enough from the finish side, or do I need to put a dam on the inside (not sure what) and then fill from the finish side with thin viscosity CA?
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Re: Hairline Cracks in Macassar Ebony Back

Postby Barry Daniels » Thu Mar 02, 2017 10:54 pm

The cracks look pretty big so I would advise medium viscosity, and with that, you shouldn't need a dam. It might take several applications of the CA to build up to the surface. Let the first application cure before proceeding with additional layers. Keep going until your glue stands proud of the surface at all points of the cracks.
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Re: Hairline Cracks in Macassar Ebony Back

Postby Philip Secrist » Thu Mar 02, 2017 11:14 pm

Thanks Barry! Hey, I had someone suggest that I strip the back finish off, apply clear epoxy to fill any cracks (and potentially prevent further cracks in the future), possibly even put CA in the known cracks as a double precaution, sand the epoxy down, and then refinish the back. It kind of sounds drastic, but I wondered what you thought. There is one additional little crack above the knot in the second picture. That one wasn't there a year ago when it was built. The only concern is that the back thickness is 0.070 in. to 0.080 in., so I wouldn't want to take off too much wood in the stripping, epoxy, and sanding process to get ready to refinish. It is a beautiful guitar and I would like to give it the best chance of long term survival. It is just over a year old. Of course, I know that there are no guarantees, regardless of what repair I do.
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Re: Hairline Cracks in Macassar Ebony Back

Postby Barry Daniels » Fri Mar 03, 2017 12:20 am

A thin layer of epoxy is not going to stop cracks any more than the existing finish. I wouldn't bother.
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Re: Hairline Cracks in Macassar Ebony Back

Postby David King » Fri Mar 03, 2017 12:56 am

It's possible that brushing a coat of thin shellac on the inside would slow down transpiration of the wood if you're in a region that sees drastic changes in RH.
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Re: Hairline Cracks in Macassar Ebony Back

Postby Philip Secrist » Fri Mar 03, 2017 1:18 am

Thanks Barry. I think if I can get the medium CA down in the cracks, then level with razor blade method and finally spray the back, I should be okay. David, I am out west (Utah) where the humidity on average is pretty low year round. I had the wood in my shop for about 4 years before I used it. I figured it was stable. As I mentioned, the cracks I am addressing are the same ones I saw when I built it. I just thought I had filled the cracks well enough before I applied finish....apparently not! :( I do try keep my shop between 40-50 RH all of the time.
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Re: Hairline Cracks in Macassar Ebony Back

Postby Todd Stock » Fri Mar 03, 2017 5:51 pm

This is a stability issue with flat sawn wood - it'd going to crack, and you should get used to repairing these cracks. Avoid poorly selected, seasoned wood in the future. I humidify to close up the crack as much as possible, then medium and thin to get a penetrating, crack-filling mix tailored to the width of the crack...straight medium is too viscous for cracks, and thin may be too runny...stay within the same brand and type (don't mix CA Gold and standard CA, etc.). Scrape level and sand per the suggestion, then drop fill...CA screws with the surface tension characteristics of lacquer, so expect to do a couple applications, then let it dry for a couple weeks. level, sand, and buff. I use linear cross-grain cleats on the top, but still use thin, very tapered diamond cleats on the back cracks because they seem to hold up OK.

I doubt shellacking the interior is going to address much of anything on this guitar, and the film thickness needed to balance the exterior finish would be a lot of coats that are going to be a PITA to apply evenly. Further, it complicates repairs - even superglue sticks better to raw wood than shellac.
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Re: Hairline Cracks in Macassar Ebony Back

Postby Philip Secrist » Mon Mar 06, 2017 7:54 pm

Okay. I must have done something wrong! I filled the cracks with CA, scraped the glue flush with the finish (and it felt smooth. I mean I couldn't feel the cracks), then I scuff sanded, masked the rest of the guitar and shot it with coat of Nitro lacquer and what you see in the pic is what I got. The lacquer is not covering the area where the CA was applied. What did I do wrong? Did I not get the CA sanded down flush enough? I only put on one coat of lacquer. I can sand it off but what do I do to get the lacquer to adhere where the CA was applied. Help! Thanks in advance!
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Re: Hairline Cracks in Macassar Ebony Back

Postby Barry Daniels » Tue Mar 07, 2017 11:25 am

Are those depressions or raised ridges? Can you take a close up?
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Re: Hairline Cracks in Macassar Ebony Back

Postby Philip Secrist » Tue Mar 07, 2017 11:31 am

Finish Edges. There were no depressions before I sprayed and I scraped the CA flush so that I could not feel it on the surface before I sprayed. The spray coat flowed up to where the CA was and sort of conglomerated along the CA edges. Todd in the last post said the CA will mess with the surface tension of the finish and that it might take a couple of coats. Perhaps this is what it is. I have since drop filled those areas and scraped flush. I was thinking I would scuff sand again with 220 just to make it all level and then shoot it again with another coat and see. What do yo think?
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Re: Hairline Cracks in Macassar Ebony Back

Postby Barry Daniels » Tue Mar 07, 2017 12:14 pm

One coat of nitro is not really sufficient to do any filling or smoothing. I would use my artist's brush and concentrate a couple of spray streams right over the cracks and see if I could get the surface built up a bit and start to see the edges of the CA blend in. If that was successful, I would level sand the area again and do a couple of full coats over the entire back, level and polish.

If that didn't work, I would sand that back down to the wood and do it over.

Personally, I think you used to much CA on the cracks and did not get it scraped down sufficiently. But Todd's surface tension concern may also be the contributing factor.
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Re: Hairline Cracks in Macassar Ebony Back

Postby Philip Secrist » Tue Mar 07, 2017 12:29 pm

Sounds good Barry. I agree. Thanks!
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Re: Hairline Cracks in Macassar Ebony Back

Postby Philip Secrist » Tue Mar 07, 2017 12:55 pm

Hey Barry. The guitar was fully finished before I filled the cracks, the guitar is a year old. If I need to sand down, could I just go far enough to take off the excess CA glue on the finish around the cracks and not all the way to the wood?
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Re: Hairline Cracks in Macassar Ebony Back

Postby Barry Daniels » Tue Mar 07, 2017 1:12 pm

Yes, that is certainly a possibility and it is probably worth trying because you can always sand back to bare wood if that fails.
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