I have five short cracks on a 40 year-old mahogany back Gibson that were a result of lack of moisture. I've rehydrated the guitar, the cracks are now well aligned, and I'd like to know what type of glue would give me the best results and whether cleating is necessary for the inside in this case.
Although I've always leaned towards Titebond for most wood repairs, I see quite a few people using Thin CA glues for tight cracks and wondered if this might be an appropriate repair for it. As I've said the cracks have closed and are clean, it's a nitro finish (Gibson), the surface is even, and I know that the CA has "wicking" or capillary tendencies.
Crack #1 (Pictures below) is under the center seam bracing....that won't need a cleat. #2 and #3 are about 2 inches long and near the kerfing and I'm wondering if cleats are necessary. There is no movement on either side of the cracks when pushing on them (There was when I first got the guitar. Thankfully there are no cracks on the mahogany sides or spruce top.....small miracle. There are also no cracks in any of the back bracing or kerfing.
I appreciate any and all suggestions,
Thanks so much!
I've included pictures of the five cracks. The only crack that has me a little concerned is #4. It's still closing up and is the worst of the bunch. #5 is near the center brace, but not on it. It is tight and has no movement.
Some food for thought. We used to rehydrate to close cracks, work glue in there (Titebond original or Titebond extend...), level with rare earth magnets, let dry, cleat, bill, next.
We've learned though that for cracks that won't stay closed say three days later at 45% RH that we induce more stresses in the plate somewhere else by forcing cracks closed that will not stay closed under typical, normal RH. Of course this is good for business but not good for the guitars and that's all that really matters to us anyway, the guitars.
These days we rehumidify for no longer than several days and then if the cracks have been open for any length of time we let the instrument acclimate at 45% for a couple of days and then decide if we are gluing and cleating or filling and cleating.
It's been my experience that the older and longer time that a crack has been open the more it becomes a candidate for filling so as to not create stresses that will likely crack somewhere else.
It's been interesting to observe this process because it's often the case that we can get something to close that's been open a while but it's not always that we can get it to stay closed at normal, typical RH. Hence the filling.
Thank Hesh....very good to know!
This is my experience too, good post. I might add that hide glue is the best glue for top cracks. It will suck the two surfaces together when drying (Titebond won't do that). It's almost invisible, strong and very easy to clean up.
My soap box two cents worth...
Why crap up a glue joint with an adhesive that is difficult or impossible to cleanly remove? If you have to re-visit the problem you will regret the extra work you have created for you're self.
I won't use anything else than hot hide glue for crack repair.
Thanks Paul and Roger....Will the hide glue be able to get into a tight crack without watering it down?
Well repaired cracks should not have to be revisited.... The guiding principal that we use for when to use HHG is does the joint have to be "serviceable" as in having the ability to be taken apart in time. There is more too with me in terms of sonic transparency but that's subjective and I won't go there generally.
The context here may be someone who may not be very experienced with crack repair. Advocating HHG with very short open time, either preheating or hot room requirements or both to use it is not the advice that I would give someone who is not experienced with HHG.
We use HHG frequently and when I was building I exclusively used it for braces, joining plates, bridge plates and bridges.
If we do not work in a hot room, use a heat lamp, preheat parts or all of the above HHG will often jell before the job is done. If not doing anything to extend open time you really need to have all clamps in place in less than a minute and shorter depending on shop temps and gram strength of the glue. I always used 15 seconds as my benchmark for having clamps in place with work that was not preheated.
There is also the issue of "leveling" a glued crack. Extra open time that other glues other than HHG provide is desirable when we want to not have the glue "tack" before we can do what ever we want to level the glued crack.
I use HHG at times for cracks but if time to get everything right is required, and it will be with someone who does not do this often other alternatives reduce stress, increase working time and as such are more likely to result in success.
There is also the question of what someone has. Doug never indicated that he has HHG.
And Roger again the term "best" is nearly always going to be questioned because "context" and applications vary widely. Defining terms is helpful too when other folks may actually take your advice and act on it. By HHG do you mean what we call HHG in the states or what we call "pearl" glue here that has other animal parts beyond hides and is widely sold as HHG in Europe? Do you mean that awful Franklin bottled HHG or good old fashioned HHG in a specific gram strength?
Not trying to be a dick with anyone here. It's simply important to me to be clear and never make all encompassing comments that this or that is always the "best..." It rarely is.....
Hot hide glue (that's what I was talking about) has a reputation of being difficult to use. I don't think it is hard at all once you've tried it out for a while. And reading about it before trying the first time. There is a learning curve for sure, but once you get the hang of it it's really simple. I recommend any beginner to take the plunge.
When doing crack repairs I glue the crack in one pass making sure the two sides are level and the crack is tighten. When dry enough (takes about 15 minutes or so if I'm in a hurry, usually I do something else for a while) I glue the cleats. No need for clamps on the cleats, just place them and the glue will do the tightening! Before the first gluing I heat the wood with a heat gun at low enough temperature not to hurt the lacquer, with the spirit varnish I usually works with I can go a lot hotter without damage. With some heat and a little bit of forward thinking there is plenty of time to glue a crack.
The main benefit with hot hide glue is that you don't have to be careful at all, you can make a total sloppy mess with the glue doing the work. It's easy enough to wipe it all off with a damp cloth. Even when the glue is dry, it only takes more time. If you don't get it right the first time you can redo it without problems, try that with any other glue...
Hot hide glue has one minor drawback, and that's the short open time. I don't use hot hide glue when I glue back the bottom due to the long and thin sides/kerfing and the time needed to match the bottom to the sides. Luckily there is fish glue that has all the properties of hot hide glue and a long open time. Fish glue is not exactly as strong as hot hide glue and probably a bit more sensitive to moisture (I have yet to see some real proof of that) but is a really good substitute.
I use hot hide glue not mainly because it's the best wood glue there is, but because it's so easy and forgiving to work with ;-)
I haven't worked with HHG before...you're correct, but I always research whatever I'm doing first and then test it out on a similar scrap of wood...I am methodical, if nothing else and don't think that I will have too much trouble getting the feel of its properties. I am most grateful to you guys for the insight and find this forum to be a wealth of knowledge that is quite important.
In my case the cracks are completely lined up and, except for crack #4, are all tight. My only question at this point is ...should all "back" mahogany cracks be cleated and if not, what is the threshold for cleating?
Talking about glue in general, hot hide glue is not the "best" for everything. It's my main glue and I use it wherever I can. For crack repair it's the best choice IMHO.
But I use CA often for small quick things, for frets, plastic bindings, for wood dust fills and to strengthen wood around screw holes. I use original white Titebond when I do brace repairs through the soundhole and also when gluing big patches of veneer over damaged thin sides, hide glue is so strong it can buckle the side from the drying force! Epoxy to glue carbon rods and sometimes on major things like broken necks.
Cleats are not really necessary if you do a perfect gluing of the crack with hot hide glue. It will be stronger than the wood and a new crack will come in another place. But it feels great to do them as a safety precaution. Hot hide glue don't fill out voids, if the crack is not completely closed a cleat is needed.
Great advice, Roger.....you've answered my question.....Thank you!
Hello Hesh, thank-you for the posts. I really appreciate your thoroughness in your description of an issue. I always learn a lot from your discussions.
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