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When handling this finished-thickness Sitka top when bracing, it cracked a few inches in from the edge of the lower bout at the maximum width on the bass side. I opened the crack, put in a bit of Titebond, and added two cleats. I finished the bracing & closed the box. Shortly after the guitar was finished (french polished), I discovered a hairline crack on the treble side (see photo). This crack is exactly the same distance from the centerline (+/- 1 mm) as the repaired crack, which is just barely visible. (You can feel it better than see it.) So it would seem that there was a weak grain line in the book-matched top pieces. The two hairline cracks have been stable for more than a year.

A few questions:

Has anyone found weak grain lines that crack easily in a top? For example, has a the top cracked when you check for cross-grain stiffness? Is this a common occurrence?

Because the second crack appears stable, should I let it alone or try to repair? If repair is recommended, what is the best approach for such a fine crack?

What's your set-up for controlling RH (relative humidity) in your shop? This is the time of year where your heating systems are on in PA and where I am Michigan too. If the wood was not maintained at a stable, acceptable RH during the build I would find that far more suspect than any defects in the wood itself.

Independent of the RH, the one side of the top cracked before it was braced just from handling - no tension from bracing or box closing. The other side cracked along the same grain line as the 1st, as far as I can tell. So I suspect a weak grain line.

However, I have some info on RH (sling psychrometer):

Bracing RH 42% May 19, 2013
Boxed closed RH 43% May 27, 2013
Note on build log: "Hard to control humidity. Swings 38-46%." I assume that this refers to swings occurring between bracing & closing the box. The critical chores were both at the same RH, although perhaps the wood had not stabilized at the RHs based on my notes.

Sounds like you have the RH pretty much under control and understand the concept that Hesh was getting at.

I don't have a lot of experience at building but have run into what you are talking about on a Gibson I repaired last year. The guy brought it in because of a crack on the back on the bass side. I inspected it while he was right there and I used a light inside the guitar to show the crack up better. I found a matching crack on the treble side pretty much as you are explaining. Both were right next to a strip of tight grain lines so it was clearly a weak spot in the billet when the tops were cut. It didn't make the client very happy to find a second crack but I put cleats on both and told him it was good we found them before they got too bad.

I would suggest you repair the second crack soon. If you don't it will only get worse. I would force some Titebond in from the back and then put cleats on it. As for the finish, that is your call. Since it is French Polished it is quite easy to repair but it will be a repair none the less.

So I would have to say yes it is very possible to find weak grain lines in wood. It is wood after all...

Bob

Thanks Steve and great job of both RH control and keeping records too. Nice that you have the sling also - excellent and accurate way to know what the RH is.

The swing in RH you recorded a bit on the low side would not concern me because it's minor and also on the low side. Many builders prefer to build lower than 45% and it works well under the right circumstances.

From the mirror like/book matched appearance of the second crack sounds like a flaw in the wood as you suspect. I've not had that happen to me but I have noticed more runout than I like after doing the rosette.... That top got burned....

As to how to repair. If needed humidify the instrument until the new crack is closed on it's own. Once closed with a hand in the box or a curved stick used to act as a jack with a fulcrum inside the box if it's too far back to reach completely manipulate the crack open and shut, up and down while with the other hand rubbing in glue from the top. If you want to relax regarding the short open time of HHG Titebond original is fine for this. Glue wise you want to be able to see the glue on the inside of the box, hopefully little beads tracing the crack's path.

Cracks in wooden instruments are a magnet for dirt on our fingers and then it gets glued into the crack and is counter to the goal of an invisible, stable repair. I always wash my hands before gluing cracks because of this.

Once you have glue in there and it's closed completely on it's own accord either naturally or due to humidification and while the glue has not jelled cover with waxed paper and then something mostly flat but with some flex. 1/8" plexi works, lots of things work if you give it some thought. I like clear plastic for positioning benefits. Then clamp flat with either long reach clamps or I use rare earth magnets the 3/4" ones with at least 40lbs of pull.

I typically cleat the next day and do it in two operations mostly because I use magnets and they can jump and hurt you or worse...... the guitar.... so not cleating for me at the same time is a bit safer.... since I use magnets for my cleats too.

If done well and I am sure you will do it well the crack should be level afterwards and a couple wipes with warm water on a paper towel will remove any glue residue on the top.

Then if necessary you can hit it again with the FP which is the beauty of FP it can always be added to.

Hope this helps and it sucks that the top wood likely had a defect. It happens to us all. If that crack continues to be on the move and the instrument is in a stable, proper RH environment I might suspect since I can't know if it was well seasoned wood too.

Bob and I posted at the same time and agree! Time for a cyber beer! I'll buy and coming from me that's something.....

I had a similar thing happen. A few months after putting on finish on a sitka topped mahogany dread I noticed what I would call a misalignment of grain lines, one on either side of the upper bout. At the time there did not appear to be any crack but one side did eventually crack. Most likely the other will also. At the time I put it down to weak grain structure in the top. Not sure how one can get around this type of problem................the top appeared sound in the raw state. Gave the guitar to my grand-son to use as a beater.
Tom

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A person who has never made a mistake has never made anything!!!

Most likely the issue was already in the top, a seasoning crack in the billet and even the log before the top was processed. I am not saying that the vendor even knew about it because they are not always really noticeable unless the top is "stressed" (deflected) before it is braced. I have seen this at times and in my opinion it is not an issue related to humidity in this case. Sorry that your discovery was so far along in the building process...

Shane

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Canada

Since I am a relative newb to building I am wondering if "candeling" tops and backs prior to building would be of any value in discovering these faults. I have been using a light inside repair jobs as part of my inspection process especially if it is a crack repair. When I first started using it I was amazed to find what I thought was a little 4" crack turn out to be much larger than I first thought. As I said in my previous post I have found cracks that my clients didn't even know they had'

Does anyone candle their wood prior to building?

Bob

I had the same thing happen recently on a Sitka top shortly after it was finished but before it was set up and stressed. I think there were weak areas waiting to crack. It may have been coincidence, but it looked like there were faint dark lines where the cracks formed--on one side, there was a line but no crack yet. There were a few other things that were suboptimal about the top (I'd ended up getting thinner than I'd wanted, etc.), and when the symmetrical crack thing started to happen I decided to replace the top.

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Tim Allen
"Never hurry, never rest."

Weak spots along grain lines are common in my experience, - having made furniture for 35 years.