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I'm wondering if anyone has had a similar occurrence. I'm trying out , on a new build...a WB Lacquer, EM6000. It's been going on fine and I like it so far...but I'm getting up now to the last several coats on the body. It's hanging there in the shop (controlled to 40% on the hygrometer)), and I see as the sun is reflecting on it just so...that there is a 3" line showing on the back near the edge of the lower bout. under the finish....no crack showing / transmitting up through the finish , but a minute depression in a thin line looking like a crack under the finish. If I put my hand in and counter push back out ever so slightly, it goes away...till I take my hand out.

Any thoughts.? Back was originally sanded to .100 thickness.....it's H. Mahogany.

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Photos?

Sorry Robbie..it's impossible to photograph. Largely invisible....but unstable. Further inspection shows a hairline crack inside.

CA. glue repair? From inside.?

 Glue joint or no?

No Kerry.....2 pc. Back.

My gut reaction answer. Glue the crack and cleat (if really needed) If you have a finish depression, then fill level with surrounding finish. Now back to final finishing. If you have to apply glue from the inside, my choice would be hot hide glue, and rub into crack, then clean up with a damp sponge or rag. With CA you may have to scrape off dried excess glue, to make it invisible.

Things happen but there is always a solution. This sounds like a minor issue.

Jim

Put some Naptha in an eye dropper and drop it on the crack (from the outside) if you can see the wet spot... on the inside it's a crack.  If not it's possibly a scratch and it's not uncommon for the pro finishing guys to see these during finishing and what they often are are a missed scratch from the drum sanding process that didn't get addressed in final sanding.  

I'd still be concerned as to why it happen though if it is a crack and would take a look at what I'm using for a hygrometer.  The vast majority of digitals can be way off.....  If you have a decent hygrometer and it's been checked against say a wet bulb test disregard but it's the first thing that I would look at not knowing your set-up.

Thank you Jim..., and Hesh ! I came to similar thoughts around 2 am this morning....but your advice,....I will follow to the letter. I had a feeling closing up the shop last evening, maybe it's too dry...so i bumped up the dehumidifier to 50% from 40. I just looked it over , and the visual evidence of any crack / problem disappeared. But ,I know it's there. So.

My Hygrometer is junk......it's time to invest in a proper unit, to get that sorted out. Any recommendations?

I will run some glue in the area of the crack...hide glues sounds good ( but may not get much in , as the crack doesn't open much, and I don't want to push things), so.. I may wick some thin CA. in, and scrape. Probably won't cleat it, as I see things now.

Again ...thank you both !

Michael

CA will work, just more messy and harder to clean up that hot hide glue. If it's a tight crack CA will definitely wick its way in and work fine.

Jim

Thanks Jim. CA it is.

Michael, aren't you here in So. California where our RH was in the teens a week ago? 

Anyway, It seems to me that H Mahogany is more sensitive to humidity than a lot of the other woods I've worked with. I've had problem with one solid mahogany guitar that has, twice now, split when the Santa Ana conditions crash our humidity very quickly. In my case the problem is in the thinner top but I had one in the back too the first time it happened.

All of these cracks were in the instrument when I got it. I repaired and cleated all of them with clean results except for two which didn't close up as well as I would have liked so I splinted them before I cleated them. Those two have remained stable while the cracks that I did not splint, have reopened twice now. This has me thinking that I should splint these other cracks too. I'm thinking that the splints will allow some room for the wood to move enough to resist cracking in the future. 

Hi Ned....No, I'm in central Vermont. The crack evidence seen from outside...just barely...is gone now. The finish was never affected. I carefully dropped some CA glue in the area of question (inside)....with several pipettes joined together, to make one long pipette, and the tip stretched out to apply sparingly. I was able to sand  / scrape the surface inside pretty well afterwards. It was an unhappy moment initially....but as Jim predicted, it was a minor issue. And I learned something . Although I DO need a proper Hygrometer....I noticed that the sun comes in the window (near where it was hanging) at just the right angle at one point in the day...for about an hour. I believe the back was exposed to this and maybe caused my dilemma. Maybe.

Thank you all.

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