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!

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Yes.  Shellac is a great sealer. It can marry many disparate materials that otherwise cause adhesion problems. Nitro doesn't much like poly, CA,  epoxy or silicon and shellac will make them friends.  I always use it and, if you're doing a clear finish, it adds a lovely vintage vibe.  Canned shellac is okay if you make sure it's dewaxed and you check the date. It needs to be fresh.  I always mix my own using fresh flakes from and Everclear grain alcohol for the solvent. has a lot of information for newbies, though you'll have to browse a bit to find it. There is also copious info on YouTube.

From what I can see, you built with flatsawn wood containing a wide area of sapwood (or a light heartwood area that looks like sapwood?).  There's a good reason that such wood was, until very recently, considered unfit for guitar building. And it developed multiple cracks before you were done finishing.  I would be worried that the problem will be ongoing.  How level is the back now?  How about a couple of photos that show the whole back?


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