Cracks on a new build

[Justin Tugaoen]

I've got a figured Narra acoustic I started in 2011 working to the point of closing the box. I got back to it again in 2013 when I noticed several cracks mostly along the grain on the back, none in the sides, as well as some irregular flattening of the radius. I sent ahead and took off the back, repaired all the cracks with super glue and several cross grain cleats along the larger cracks (up to 3 inches long). And re-braced the back using slightly thicker bracing this second time around. I then finished the binding, endgraft etc and waited a year to see if any new cracks appeared. Aug 2014 I didn't really notice cracks and the radius was stable so I pore-filled and sprayed ktm9 finish. So now 8 months later I flat sanded to 4000 grit and now I notice new cracks in the finish! Not really noticeable from the inside, but at least one of them in along an old repaired crack and one is along the center seam. Unfortunately I did not use a center inlay/binding, just the inside cross grain bracing. What to do? IS narra inherently unstable? This is one of a batch of 5 acoustics, including bubinga, Spanish cedar, Indian rosewood, and spalted mango, but none of those have any cracks. I worked on all at the same time, kept them all in the shop throughout (basement shop) so I'm not sure what to make of it. Suggestions on how to repair? I buffed out a small section so the crack shows up better in the picture. Thanks for your thoughts.

[Justin Tugaoen]

Here's a pic of one of the cracks

[Michael Lewis]

Could be some issues previously in the wood. I usually test wood with a thinner that will evaporate completely, as it shows up the cracks as the uncracked areas dry. Naphtha is a favorite if you can get it. Hard to come by in California. It could be the wood wasn't as dry as it should have been when it was glued up. Do you have a moisture meter?

[Barry Daniels]

Do you control humidity in your shop? How long did you have the wood before you started building with it?

[Justin Tugaoen]

I had the wood back and side set for at least 4 years prior to starting the build, stored also in the basement. The humidity in my basement shop varies usually between 40- 50%. No I never check wood MC. So would you suggest sanding down the finish, repair the cracks again then respray? I'm starting to feel like it's just going to keep moving and cracking. This guitar is just for myself and the kids btw.

[Barry Daniels]

I don't think that removing the finish will help. Just repair the cracks, cleat and keep the guitar hydrated.

[Justin Tugaoen]

Thanks for all your thoughts and input! Because I couldn't stand the thought of finishing a new guitar and already have cracks on it, I went ahead and removed the finish and sanded it.....but I didn't really see any cracks that needed repair! The center seam was palpable but no real visible cracks there either. So all I did was put some thin glue over the center seam section (approx 10 cm long centrally) and sanded flat.
So do you think maybe a sprayed too thick of a finish? I figured wood will mover no matter what and the finish would just move with it to some extent but not crack like this. (I used KTM-9, approx 12 coats total over 2 days). From what I can tell the back still has a fairly smooth 20m radius to it and no obvious flat or sunken areas, but got cracks nonetheless.
I resprayed 12 or so coats of KTM-9 as before and just waiting for it to dry and buff again.

[Michael Lewis]

What application schedule and rate does the manufacturer advise? I have a suspicion you are putting too much on over too little time.