Hairline Cracks in Macassar Ebony Back

[Philip Secrist]

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 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 if 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!

[Glen H]

What I would do - make sure the guitar has not dried out.  Store it at 45% or so for a couple weeks and see if they change.  Then work some hide or titebond glue down into the cracks.  Let dry a couple days.  Scuff sand and spray a couple coats  finish on the back.  Tape off right on the corner of the binding and spray the entire back.  The corner is a natural break.  Level and buff when cured.  One thing I avoid at almost any cost is sanding through the finish and then trying build up finish on the  local bare spots.

 

[Greg Mirken]

Assuming the wood was stable when you built, and there aren't other signs of it having dried out [sunken top, etc.] I would use medium viscosity superglue on the cracks. Definitely refinish the whole back so the fix doesn't show.

[Philip Secrist]

Thanks Glen and Greg. There aren't any other signs of it having dried out.  The crack really  looks the same as it did a year ago when I finished the guitar.  I just thought it was an improperly filled grain line so I leveled and buffed it out. It look great after that, but It is definitely a crack and a year later looks the same as it did before I leveled the finish a year a go. I can't see any crack on the inside of the guitar, so I don't believe it goes all the way through the wood. The crack is pretty tight and doesn't seem open up when pushing on it from inside. I am not sure how to get hide glue, titebond, or medium superglue into the cracks.  Would light viscosity super work better?  Any suggestions there? If I could get hide glue in, I would prefer that since it tends to close the gap better, but when I built the guitar, I did put some superglue in the crack thinking it would fill it,  so I don't know if that would adversely affect  hide glue or titebond if I tried to fill it with that now.  I plan to avoid sanding through and will refinish the whole back.

[Roger Häggström]

I'm no expert, but I'd use thin superglue in the crack to get it in there and put a cleat glued with hide glue on the inside to secure the crack for the future.

[Russell Vance]

Hi Phillip,

Plus 1, what Greg said, those bad boys need to be filled and thin CA will just keep wicking in so you will need to dam the other side to get a fill. The medium fill will go off as one layer and a final application build to scrape down.  Complete refinish and just mask up to the corners as the buffer will take out the edge after a rough knock down with micromesh or similar. 

If it was my job I would also cross band some laminate inside the box to stop any further movement.  I'm not a fan of cleats when you can literally place a full sized laminate/patch or strip  across the problem area.  The reasoning here is that cracks never get smaller over time and nor do the tensions that cause them in the first place.

Good luck with this and tell us how it goes,

Rusty. 

[Roger Häggström]

I use whole strips as cleats too, at least on the hardwood bottom. On the sides I use maple veneer. For the spruce top I use elongated cleats (about one inch long with V shaped tips) in a "caterpillar train" to cover the whole crack, each cleat placed a bit zigzag to the one before and after not to form continuous edges from the sides of the cleats as a starting point for the next crack. I also have a bit of space between the cleats to give the top more flexibility.

The added weight from a continuous strip compared to a couple of standard cleats is very small. That extra weight may be important on a violin, but not on a guitar. Violin repairmen even cuts a diamond pit in the top and fills the hole with the cleat to keep the exact same weight!

[Philip Secrist]

Thanks Rusty!  I have since confirmed that the crack goes all the way through. I have not used full sized laminate/patches, just cleats on other jobs..  The back wood is a Macassar Ebony. What do you recommend for a laminate patch, how big, and how thick? The crack is about 3 inches long and a little curved.

Thanks a ton!

[Russell Vance]

Hi Phillip,

Couple of things, given that you will be gluing the crack together its not neccesary to have an equivalent thickness of the back in the patch. 

Given that the repair patch is cross grain oriented its not necessary to make it out of the same wood as used in the back - mahogany or walnut is what I use generally because we have a lot of it pre thicknessed at various dimension for laminating repairs to necks etc.and it glues and stains well and has a bit of flexibility.  

A cross banded (grain orientation at 90 degrees to the crack - or take the average of the curve and orient the patch to cover at a average angle) 1/16" solid patch or curved strip is OK and providing the back isn't too bowled can be glued in place with a caul and magnet arrangement.  I generally put a few tape tabs on the strip to initially locate the patch and stop it moving around as the caul is manoeuvred in place.   Whatever works is good here but it's a Titebond repair as you may need some working time fiddling around in the box.  However, beware that Titebond applied on one side of a veneer or laminate will cause the wood to curl a bit so what lays flat when you "dry run" it won't stay flat when you apply the glue - work fast is good.

Have some fun and a good repair,

Rusty.

[Philip Secrist]

Thanks Rusty.  Would you recommend against using hide glue to glue the strip.  I don't expect it would ever need to be removed, but it could in the future with hide glue, right?  Thanks again for your help!

[Russell Vance]

HI Phillip,  Hide glue is fine,  Titebond just gives me a little more working time because I can be a little clumsy while working in the box.    R.

[Roger Häggström]

Fish glue is a great alternative to hot hide glue. Almost as good but with a long open time. A bit stickier, viscous and more sensible to humidity compared to hot hide glue.

[Philip Secrist]

Hey Rusty,  just to clarify, are you saying I should dam the other side and use thin CA or use medium CA and not worry about a dam inside?